The most overrated Super Sentai series is undoubtedly Dengeki Sentai Changeman. The main characters have no personality and Commander Ibuki is a complete asshole. As a matter of fact, he's the true villain, as he led the Gozma Tribe to Earth after they destroyed his home planet. Also, what's with the sports bras over spandex as their suits?
Oh yeah, Changeman fans need to lighten up. I mean, if people hate Shinkenger, Dekaranger or Gokaiger, but that's their thing and I won't hold that against them. Changeman fans, on the other hand, have a tendency to go ballistic on people who simply don't like the show.
As for underrated Super Sentai series, definitely Battle Fever J. Behind the silly-looking costumes and the ridiculous dances is an excellent cast of characters with great personalities and a badass commander.