Kakuranger, Megaranger, and Liveman. If I had to take it out to five, I would add Maskman and Jetman in there, but the three are my solid choices.

If I had to make a top 5...
Kakuranger
Megaranger
Zyuranger
Liveman
Jetman

But then again, the only other one I've seen is Dairanger and this is my least favorite Sentai, together with Boukenger. So yeah.

It's hard. Older sentai is a bit too monster of the week, newer sentai is way too flashy and fast paced. I've been focusing on the middle stuff mostly because of that, and have been having some luck (I seriously recommend Jetman if you haven't watched it yet) but i'm not really sure what to do after i'm done with these. I'll probably go old school because i'm mostly interested in Toku because of the suit battles - CGI is a big reason why I can't get into newer stuff. It's understandable that you'd prefer newer though

There are more post-90s sentai than there are pre-90s, so i'd say it's safe to consider it "old school." Well, maybe Gingaman is pushing it a bit, but I don't think you could argue that Jetman, Zyuranger, and Dairanger aren't old school.

My top three:

Jetman
Kakuranger
Megaranger

(No particular order)

I don't feel like Jetman was that good of a series, to be honest. The events that it's remembered and commended for only occur in 5 episodes out of 51. It has very little substance outside of Gai's brash attitude, which he drops after the first 5 episodes beyond a snarky remark here and there.

My biggest beef with Jetman is how Kaori became a prize for the male members and Ako was kind of forgotten. Raita had a girl that wanted to be with him, but he fought for Kaori, Gai fought for Kaori, and Ryu rejected her (even with all her attempts) until he watched Rei die. Then they got married at the end of the season.

I think if the female characters were better written, it could have driven the story a little more. The villains really progressed the story, IMO; kind of like how the Shadow Line progressed the story in ToQGer.

I haven't watched many of the of the old school sentai's but the ones that I did like are liveman, kakuranger, maskman, to a degree zyuranger, and was gaoranger in the 90's?

No, Gaoranger is from 2001.

I noticed that too, which is funny to me, since I find Ako both more enjoyable as a character AND more attractive.

Well, let's look at Megaranger, a series I personally found to have very little substance. Red acts like an idiot, everybody else takes everything more seriously. Then they fight the enemy. If the enemy is Yugande, he whines and then has to come up with a way to redeem himself. Rinse and repeat. I only made it about halfway through because I got so bored I couldn't finish.

Jetman is interesting to me because of the character dynamics. Raita and Gai are both into Kaori, who is into Ryuu. Gai and Ryuu are constantly at odds with each other, both over Kaori and the Jetmen in general. Poor Ako is left out. The villains are all also trying to one-up each other the whole time. I like the dynamics, and haven't seen any other Sentai with as much character drama. (then again, I haven't really seen a lot. Zyuranger is the only one i've completed, while Jetman and Megaranger are the only other ones i've made it past episode 20. I've seen 9 episodes of Kakuranger, 6 of Gokaiger and Battle Fever J, 2 of Dairanger, and 1 of Goranger, JAKQ, Timeranger, Carranger, ToQGer, Ninninger, and Gekiranger.)

I only really named Megaranger because the question didn't leave me much room, honestly.

I found Jetman's dynamic to be too intrusive. The love quadrangle got grating and it felt like the characters were too oblivious (And Gai's declaration of love to Kaori's face and then her proceeding to just string him was awful). Gai loved Kaori, Kaori loved Ryu, Ryu can't get over Maria, and Raita randomly loved Kaori, too, but they bring it up a whopping two times before it becomes irrelevant. The problem was that they basically made the characters' stories revolve around one of the most dull characters in the franchise. Kaori is such a vapid stepford smiler whose barely has anything interesting going for her.

The villain dynamic was more interesting, but it was wasted, since once Tranza got into power, it was just a giant downfall from there. I enjoyed all 4 of them, but Grey didn't do much until his end and Maria felt like set dressing. Radiguet and Tranza were the best of the two (Radiguet more so). But Radiguet didn't really get interesting until his brief time as a human and Tranza was just more of a mover, rather than an interesting character.