I think each Sentai has their own bright spots, even for people who aren't fans, Boukenger has bored me to tears and I think it along with Go-Onger and Dekaranger are the worst shows of the last ten years or so, but I still loved Eiji's entire story and he stood out to me as a really great character in a bland series.
Timeranger, Abaranger, Magiranger, Gekiranger, and (so far) Shinkenger have been the stand out series for me, with Gaoranger, Hurricanger being just "there" and not too remarkable in any way.