DancingAlienDude did a similar post, I agree with Lightspeed and SPD he covered, but I felt that there are more than just those seasons that are underrated.
Power Rangers Mystic Force
Surprised by this one, aren't you? I certainly am. I've underrated this season before, only because it focused on Nick too much. But I looked over the first ten episodes again and, it's okay. The plot and story for this season is a little more difficult to understand then previous seasons. One of the main reasons why this season is underrated because it's a Disney season and 'butchered' Magiranger, as fans like to put it. I think one of the reasons why Mystic Force is underrated because this season is a little difficult to understand because past Power Rangers seasons have been simple. You understood the story right from the beginning. Even Operation Overdrive was fairly simple to understand. This story has to be one of the most defined and also one of the most fun, especially if you are a fan of fantasy shows. I think one of the things that distinguishes this season from past seasons is that it is not really the most action filled season out of all of them. Mystic Force is very much based on mostly story.
One of the issues with this season is that the acting is really not that good. Firass Dirani, Nic Sampson, Melanie Vallejo, Angie Diaz, and Richard Brancatisano who play the main rangers are not really that talented in the acting department. They do have some moments of good acting especially in the season finale "Mystic Fate" but most of it is just hard to watch. Another issue is the Red Ranger focus. There was just too much revolving around Nick and for me, I didn't like it. But I learned that it doesn't bother me as much as it did before.
Power Rangers Ninja Storm
When I first heard about Ninja Storm I was very excited. I thought it would be dark, mysterious with action packed fight scenes. However when I started watching it i soon found that this wasn't the case. PRNS was too comical for my liking back then, the villains were to slapstick comedy funny and the rangers themselves seemed too laid back all the time, that's why it's underrated. Looking at Ninja Storm now, it's pretty good, actually. Because if you're a fan of MMPR or happen to like MMPR, you'll like this too. A villain with strong cynical intentions like Lord Zedd would have been great in this though.
Power Rangers Lost Galaxy
Lost Galaxy is probably one of the most underrated PR series due to it just coming off of Space and starting the standalone series. Despite plot holes it continued some stuff In Space started: teamups with past alumni, sympathetic villains, and the grand feeling of a space adventure. Sure it may not have that end all feeling that Space had, and no matter how many plot holes this season had, I LOVE this season. One of my favorites and made me continue watching the show today. I think there are a lot of reasons, but a big one would have to be Trakeena, who's pretty much my favorite non-MMPR villain. I was honestly kind of sad when she took a nosedive into crazyville; it's hard to imagine that she went from being a fairly background sort of villain to getting so good at swordplay that she could take down two monsters at once.
Power Rangers Jungle Fury
Surprised by this one too? I'm not. Because this is probably the best and my favorite season Kalish produced. Main reason why it's usually underreated is because it's a Disney season and 'butchered' Gekiranger. The writers strike is one reason why this season is underrated, there were a few episodes that just didn't seem to move things along, or tried to hard on the message. There was a rumor that Chip came back and wrote a few episodes, which makes the season a little more enjoyable. But the season was a success for me. In a few ways, it almost seemed to pay tribute to the old days of Power Rangers, with RJ inventing weapons or working on them, and the Rangers actually having to train and master skills before getting new levels or gear. The grouped story arcs played out very good. The Jarrod and Dai Shi conflict was handled well. I enjoyed that. All the characters were very relatable, and at times I found myself being just as interested in Jarrod and Camille as I was the Rangers. The season isn't bad, it's just people are biased.
So the next time you see a Power Rangers season that you think 'butchered' Sentai, don't complain about it, or how it doesn't work because of such and such a reason. Enjoy PR for what it is...it's own show with a very interesting over arcing tied story.