I've seen very few Sentai series, haven't gotten around to watching most of the older ones.
Gokai Silver immediately comes to mind, he's so fun and hi's suit is awesome.

My favorite 6th Sentai Ranger of all time is DragonRanger, KingRanger, GaoSilver and Go-OnGold

Genta/ShinkenGold. To me, he had the biggest impact to his team. The series was a little lifeless and cold before he showed up, and really brought fun and a great team spirit with him.