I have only ever seen 1 Sentai series, and that was Samurai Sentai Shinkenger.
Power Rangers, is a very different story:
MMPR: Cheesy, not as good as nostalgia would have you believe, but Bulk and Skull are. It established good characters and the universe, and gave us a lot of good. Overall: Okay.
Zeo: Better than MMPR, with flaws here and there, but with new costumes, and a Magic vs. Science theme continued, as well as the Rangers representing a balance between the two, it works out great. Better than MMPR.
Turbo: Replacing everyone was unsettiling, and while I grew to like the new characters, it was still odd. Divatox was a huge step down from Zedd, Rita, or Mondo. Havoc was cool though, he should've replaced Divatox. The finale was really epic, though.
In Space: Do I need to even say things? Easily one of, if not the best season, with good characters, on both sides, a solid mission and plot, and the Mega Voyager, one of my favorite Megazord designs.
Lost Galaxy: Not as good as In Space, but okay. Villains were more interesting than the heroes, in a lot of cases. Villamax was especially great. Like Ecliptor from last time but with less "Father" and more "Knight" in his honorable ways. Better than some other evil Knights of "Honor." -_-
Lightspeed Rescue: Almost the opposite of last time. I found the villains here to be dreadfully boring. The Rangers were all cool, though, especially with the Titanium Ranger arc, and etc. Not much to say with this one, though, Something new, and with an okay execution.
Time Force: Easily in my top three. All the rangers were great, even Lucas, to an extent. Villains were all really great too, But mostly Frax. Frax was the best. I will here no words to the contrary. Just all around good, no real complaints on the major scale.
Wild Force: It got over the top in some points, and there were some issue, and the Kite Arc was terribad, but I still think it's underrated. I liked it, and could recommend it.
Ninja Storm: Similar problem to Lightspeed Rescue, with the villains. But instead of just being bland, they were trying too hard for comic relief. I never liked Lothor, or any of them outside Zurgaine. The Rangers, Cam especially, all had their good points, and I enjoyed watching them mostly, but they had their poor moments, of course. Sensei sucked, though.
Dino Thunder: Not in my top three, but I'd place it at 4, probably. While yeah, Tommy as great, this series had a lot more good than that too. Trent was cool, Mesogog was a badass villain. Zeltrax looked cool, but was kinda meh as a character. The core rangers were okay, though, but like the last few, there were ugh moments. I like this series, though.
SPD: I'd say a close 5 behind Dino Thunder, really good, but Grumm was kinda meh. As was most of his army. Piggy was terrible, but Broodwing was pretty cool. Interesting dealer dynamic with him. The Sky and Jack thing was really well done, though, and Crouger was a great mentor and warrior, in a similar vein to Tommy last season, but unique in his own styles. The focus on B Squad, and the A Squad twist was fantastic, and really gave a good retrospect on their underdog status and the like.
Mystic Force: Man, I really wish they had focused on the rangers, and fixed Koragg's mess of a character. Focusing on the generic fantasy and side characters was a poor choice, and really hurt the series by the end. 'Cause I quite liked Xander and Chip. Daggeron was boring, but Udonna was a good mentor. Claire was meh, but she has redeeming moments. The episode with her and Phineas and Lili(Is that how you spell it?) Was pretty entertaining and good. Overall, there's a few episodes I'd recommend, but the series is just Meh.
Operation Overdrive: No, I'm not gonna even bother with this one. Wait for the Linkara HOPR video for a good rundown of what's wrong. If you like designs of the Zords or whatever, go for Boukenger. It's actually really good, from what I've heard.
Haven't seen Jungle Fury.
RPM: Yes. Win. I enjoyed all our main rangers, even tolerating Gem and Jemma to an extent. K was great, and the premise overall was awesome. With some serious moments, some deconstructive moments, and just all around the last of my top 3.
Samurai and Megaforce I have yet to see, though.