I actually HATED this show when it first came out but I decided to check it out late last year because everyone told me how great the show was.
And it turns out they were right but I understand my hatred for it in the past.
The first season was great, introducing the mutants and their stories and how they join the team.
The second season must have been the one I caught here and there, for me it had way too many filler episodes that not an ongoing arc until the last few episodes.
Season three by far my favourite as it was when they had been revealed to world and they were dealing with the prejudiced whilst combating the "evil mutants"
Season four was OK, again too many fillers considering it had what 9 episodes and it was supposed to be dealing with apocalypse.
All in all a great show, I loved the nods they gave to the fans. Episode with the original five, the new mutants etc.