OH BOY SUPER SENTAI!
Oh hey this thread is almost a month old. Who knows if this reply will even be seen! Well regardless I have been informed of this thread's existence so let's get down to business shall we?
I'll pretty much be sticking to series I've seen, which is all the ones post Gekiranger, as well as a bit of Zyuranger, so here we go.
ZYURANGER: It's interesting as a comparison point for Power Rangers, since this is the season that got adapted to form season one of MMPR, but it's kind of lackluster beyond that. The villains are outstanding, Bandora is really fun and they all have a lot of character. The Zyurangers themselves on the other hand...are basically devoid of personality. They're super boring. The show has a cool theme of mythological creatures being the MotWs, but most of the plots revolve around "helping random children" every episode. It's not a bad series, but I never finished watching it, I got a few episodes past when Burai joined the team and fell out of it.
GO-ONGER: I'm biased because this is my favorite series. It's very goofy however, and the silliness turns a lot of people off to it, who prefer a more serious series. But its strong points are that mecha battles are actually given weight and are well done in this series, mostly due to the fact that the mecha themselves are characters in their own right. Sentai had done sentient mecha before, but they were never really characters exactly, aside from Daizyujin in Zyuranger and even he only showed up every once in a while. By contrast, the mecha in Go-Onger are always around, and the partnership between them and the Go-Ongers is the heart of the show.
SHINKENGER: An excellent series, one of the best Sentai there is. The characters are excellent, the villains are compelling, the mecha are cool, just an outstanding show all around that comes highly recommended...oh right, except there's one little problem. Kamen Rider Decade was airing alongside Shinkenger, and after finishing his journey through the alternate worlds of the Heisei Riders, Tsukasa had to journey through some other worlds. One of them was the World of Shinkenger, essentially a world where Kamen Riders don't exist but Sentai does (which bothers me a little but that's an off topic rant about an old crossover movie that doesn't really matter to this thread). There were four episodes involving Tsukasa's coming to the World of Shinkenger, two of them being Kamen Rider Decade episodes and two of them being Shinkenger episodes. It's been years, I think you can maybe get away with just watching the Shinkenger episodes but not the Decade episodes, but I'm not 100% sure which series the whole arc wraps up in or even how much Tsukasa and Kaito appear in the Shinkenger episodes outside of short scenes. Regardless, it's an excellent series that's highly recommended.
GOSEIGER: Goseiger has a really cool final villain that I won't spoil, but it's not a great series. I enjoy it alright but it's pretty average, nothing particularly remarkable outside of the final villain. The mecha are pretty bad and forgettable too. Not really recommended but it's not terrible.
GOKAIGER: Gokaiger is really great, and doesn't even require a lot of knowledge about previous Sentai to fully enjoy, since part of the plot is that the Gokaigers are from other planets and don't know who the Sentai are either. But it's a lot more fun if you do know the previous shows, since there's a lot of good cameos and callbacks and neat stuff like, the Gokaigers appeared in the movie Goseiger VS Shinkenger as a teaser for their series. The beginning of Gokaiger says that all the Sentai gave up their powers to defeat the Zangyack and are no longer Sentai, which is where the Gokaigers got their Ranger Keys from. Gokaiger has an arc where they travel back in time to Shinkenger VS Goseiger for plot reasons and make their appearance there fit, from a timeline perspective. It was actually a cool moment. But yeah, Gokaiger is also highly recommended.
GO-BUSTERS: Go-Busters is really good and broke a lot of Sentai conventions and tropes. For instance, there's almost no stock footage transformation scenes or roll calls, they're practically all done in real time. The show also had MotWs that couldn't speak and were fairly mindless, not actual characters, and also allowed for giant enemies to show up at the same time as human sized ones, which created some cool moments. The show also has some small references to MMPR, as the Go-Busters' transformation devices call out "IT'S MORPHIN' TIME" and the giant enemies are called Megazords. It's really good! Also a bit silly, as the Go-Busters all have a particular super power but at the cost of a drawback. Red's for example is that he has super speed, but freezes if he sees a chicken. But yeah, recommended.
KYORYUGER: On par with Shinkenger in my opinion, another high point for the series. Not much to say about it other than it's really good, and very highly recommended. One small note: there is a romance that comes out of literally nowhere in the next to last episode. It's weird, it's not bad, it doesn't ruin anything, but the sheer "what" factor makes it worth mentioning I feel because it was never foreshadowed or hinted at in the slightest.
TOQGER: ToQger is hard to recommend even though I personally enjoy it. It has kind of the same problem Zyuranger does: its heroes are pretty boring (aside from near the end), but its villains are fantastic. The reason why I recommend it more than Zyuranger though is that Bandora and her gang, while great and fun, weren't really compelling. They were standard "IT'S TIME TO CONQUER EARTH" villains. ToQger's villains, on the other hand, all have different individual motives and there's a lot of infighting and power struggles between them as they all attempt to undermine the others to accomplish their conflicting goals. It's really good stuff. The villains carry ToQger and carry it fantastically. Also Right (ToQ #1) ends up having a very interesting backstory that ties him to the villains and makes the final arc really good stuff too. It's hard to recommend if you want your heroes to be cool and interesting people, but otherwise it's solid.
Oh right, and then there's Wagon, the train's stewardess. She sucks. She's awful. You just gotta kind of put up with her. One other thing worth mentioning is there was a one hour crossover special between ToQger and Kamen Rider Gaim as a prelude to the Super Hero Taisen movie that was coming out at the time. It's pretty easily skippable since it doesn't have any impact on ToQger's plot and I don't even remember if there's a lead in to it on the previous episode, but if an episode ends with the train saying they're going to Zawame City, just know that's when the special happens and if it's not in the place you choose to watch it, that "After this they met Gaim" happens. Also, the Super Hero Taisen movies suck, don't watch them.
NINNINGER: Hoooo boy. Do not recommend, at all. One of the lowest points of the franchise, Ninninger is absolutely awful in every way imaginable. The villains aren't interesting in the least, and the heroes are just as bad. Basically Takaharu (the red) is the only one allowed to do anything cool ever, and everyone talks about how he's the greatest and most special and they'll never be able to be as good as him. The other Ninningers don't matter because Takaharu rules forever, losers! To make matters worse his actor is awful, his catch phrase is "I'm getting fired up!" which is said with almost no energy or passion at all. The only interesting points of Ninninger are that it had a lot of guest stars from previous Sentai series, with characters from Kakuranger, Hurricaneger and Magiranger appearing, as well as a crossover episode with World Ninja Jiraiya, a Metal Heroes character! There's also a crossover special with Kamen Rider Drive, the concurrent Rider series. Also, the sixth team member is a cowboy from America who uses a hamburger to transform, which is actually pretty funny. But yeah, don't bother with Ninninger, it sucks.
ZYUOHGER: Zyuohger is a pretty good series that I would recommend, a lot of things about it are great aside from the mecha which are kind of lame. But other than that, the usual, good characters, good plot, neat villains, and some really cool twists. The show did do a crossover with Kamen Rider Ghost, the concurrent Rider series, but I don't remember if it was a special or if it was individual episodes. As usual for any crossover outside of Decade though, it doesn't affect the overall plot so it can be easily skipped if you can't find it or if it's resolved in Ghost. But yes, definite recommend on this.
KYURANGER: I like Kyuranger a lot, so I recommend it with reservations. The plot is good and interesting, one of the best in Sentai I feel, but there's a lot of characters and only a handful of them get a focus, and there's almost no compelling villains. Also even though I didn't, a lot of people find Lucky, the Red, really annoying. I would recommend a lot of other series over Kyuranger, but I don't think it's disappointing and I would say give the first few episodes a shot. If Lucky grates on you, he doesn't really get any better so do with that info what you will.
LUPINRANGER VS PATRANGER: Ugh. So here's the deal. Keiichiro is one of the best Reds in Sentai. He's fantastic. He's a really good character who grows a lot over the course of the series, and he's a very likeable guy. Unfortunately, everything else about the show sucks. All the other characters suck, the villains suck, the plot sucks, the Sixth Ranger really really REALLY sucks (he's technically the seventh member, but "Sixth Ranger" is just a catch-all term for a member who joins later in the season), and any interesting plot hooks they introduce into the show are squandered and resolved in extremely unsatisfying ways. The show also has a nasty habit of treating the Lupinrangers as the stars of the show while the Patrangers are reduced to bumbling fools who exist to be dunked on. This was made most apparent by the fact one of the mecha was designed to be for the Patrangers, like the toy literally says SUPER PATRANGER when you press a button on it and was obviously meant to be a power up for them to give them a super mode...and then the show gives it to the Lupinrangers. And they keep it, the entire show. And the Patrangers never get a power up. It's bad, awful, do not watch it. It's slightly better than Ninninger because it has one good character, but Keiichiro is not enough to save the show.
RYUSOULGER: It's meh. Tyramigo is fun, the villains are alright, but it doesn't get particularly interesting until the final arc of the show. It's hard to recommend a show that is mostly boring for 43 episodes and then go "But the last five episodes are pretty good!" Also the Sixth Ranger sucks again. Not recommended, it's not LuPat or Ninninger bad, but it's hard to recommend a show that's merely "adequate."
KIRAMAGER: The current Sentai. It hasn't finished airing yet, so it could get really bad at any point, but so far it's great. The characters all have excellent personalities and quirks, and the villains are great as well. It also does a Go-Onger-ish type setup where the mecha are also characters in their own right and partners of the team, although they're not used as much as the Engines in Go-Onger were. It's pretty solid all around so I'd say check it out for sure, just be aware you're taking a risk since the show could turn bad before it ends, and also you wouldn't be able to binge it, you'd be stuck at one episode a week like the rest of us. :V
So yes those are my thoughts on the series I've seen and how much I recommend them. Whew!