I'm surprised noone mentioned Abaranger as being seen as a bad sentai; here at HJU, Aba is mostly enjoyed, but in other forums (like Ranger board or elsewhere), especially when it aired, I've read a lot of harsh stuff about it because of its silliness, the talking dinos, the over the top comedy, and many said that PR Dino Thunder was much better; I guess that since, Go-onger overshadowed it in silliness
About the series mentioned in the first post: Turboranger; it's pretty hard to come after Changeman, Flashman, Maskman and Liveman; moreover the series had a lighter tone, so it looked bad; Turbo is a personal favorite, notably because it has one of my favorite characters of the whole sentai franchise; Tsukikage Sayoko/ Kirika
Fiveman: same problem, and the series lacked in originality and was pretty childish ; even if the sibling team idea was fine, it wasn't as well done as it would be later in Gogofive; besides, after episode 20, I feel the story stalls and it goes nowhere during some time; and the puppets made it worse; that said, the beginning is fine, and starting with episode 37, the series becomes better
Ohranger: that series has a bad reputation, because it has suffered from behind the scenes trouble,after the Sarin subway attack; I think Ohranger is fine and has a lot of good ideas, especially when it comes about the mechas; in my opinion, the main problem is that the villains only begin to be interesting after Bacchusfundo's death (a problem I think Gokaiger has too; after all, both have an evil empire from space as antagonists)
Magi: the critics: a lot of bland heroes, silly characters (like Mandora and Smoky), some rushed stroylines (like Vancurya turning "good", or Urara/Hikaru); in my opinion, Magi has many flaws, but also many great ideas, an interesting storyline, and a well developed universe; so it's a fine series in my opinion
Goseiger: I don't like that series, that I feel can have interesting ideas (Buredoran/ Brajira, Goseiknight) but the heroes are mostly bland and most of the villains are either not interesting or wasted potential, notably because they don't have enough time to have development