Well, of course everyone is entitled to their own opinions, but the internet does seem quite vocal about being negative towards the following (at least from what I've heard):
1. Battle Fever J
2. Goggle-V
3. Dynaman
4. Turboranger
5. Ohranger
6. Gingaman
7. GoGoFive
8. Go-onger
9. Goseiger
I hear a lot of negative comments towards Battle Fever, due to first glance at the suits. I personally think Battle Fever is one of the better installments of the 70s/early 80s, so I say this show is definitely underrated.
Goggle-V gets hit on due to the silly looking concepts. Which I can definitely understand. I mean, I see some things about Goggle-V that I think are fun to watch. I do think the show does get a bit more heat than it deserves, but some of it seems to be justified depending on who you ask.
Dynaman is often ignored due to it's bland concepts, which I can understand, but find it to be really unfortunate. IMO, Dynaman has a really fun-loving and kickass cast of heroes. So I'll definitely say that Dynaman is underrated.
Turboranger is often labeled as one of those "bland sentai that brings us nothing new," which I think is too bad. I think Turboranger did many many new things than it is given credit for, and actually manages to execute things very well. IMO, Turboranger is a very solid 50 eps that really needs more fans.
Well, I might not even need to mention Ohranger, Gingaman, or GoGoFive since these last two years, they've all been getting a steady fanbase. I'm pretty sure everyone knows what Ohranger's story is. I think some of the complaints are justified, but I don't think Ohranger is overall a horrible season. Not great, but not horrible. Gingaman sadly gets heat by fans due to a false rumor (which I think started from Rangerboard last I heard). So Gingaman definitely deserves more praise than it gets. GoGoFive has been called "boring" by many fans, which I really don't understand at all. Luckily, that seems to be turning around from what I've seen.
And I'm pretty sure people know the story for Go-onger and Goseiger. I admit, I'm not a fan of neither. And I only label a show as "underrated," only if they deserve more credit than they get, which is something that I don't feel either of the two have earned (no offense to anyone who loves them. I'm sure people have great reasons for loving the two shows).