[Matrixbeast]

Oooh, oooh! Can I make a list?

Gekiranger - Has a strong cast and places a lot of emphasis on developing the villain to prevent him from being some generic guy throwing monsters around on a weekly basis. Energetic, but doesn't shy away from some powerful drama.

Dairanger - One of the few Sentai with a rather interesting series of stories that last throughout the series. Being the second series with a 6th, it gives some pretty interesting stories to meld them in without marginalizing the other rangers.

Kakuranger - Ninjas are cool! It's super fun and energetic during the first half, but after a rather heavy series of episodes, it takes a much more serious tone, but doesn't feel out of place.

Yeah, those are the 3 that I can recommend, myself. A lot of Sentai are really good, but I feel those 3 series go above and beyond.

If I can name a few to avoid, I'd have to say
Sun Vulcan and Shinkenger (Rather dull and incredibly slow beginnings, both taking over 20 episodes to really get interesting)
Kyoryuger (It can be enjoyable, but the hero worship Daigo/KyoryuRed gets is a bit overbearing. It basically depends on your tolerance)