I like seasons 1 and 2. 2, I think, is underrated. I did not like the first half of season 3, but loved the second half. I haven't checked out season 4 yet.
I do agree that season 2 is underrated, but it does have some major problems.
There's Peter's girlfriend who was lost in the future and then never mentioned again. (Real nice, Peter!)
Everyone also figured that Kensei was the mysterious person killing the Old Heroes way before it was revealed. It just became exceedingly obvious, and by the time they revealed it and expected it to be shocking, instead it was perfunctory and dull. "Who cares? We already knew that!"
That said, I did like season 2, especially considering what we got afterward...