Are sure fire skips? Basically which ones should I not watch under any circumstances?:disappoin
I can't speak for Sentai, but the only Rider I'd say is a must-not-see is the last 19 episodes of Hibiki.
In general though, every Sentai and Rider I've seen have had things going for them and things going against them; none are so perfect as to be beyond reproach, nor any so heinous as to be unwatchable and where any given show falls is going to depend mostly on your tastes and what it is you're looking for in these shows.