The first season was some of the best anime I've ever watched. The unique character of Saitama, the brilliant animation, the humor, action, epic scale, I can go on. I've watched the first season multiple times and enjoyed it every time through.
However, the second season was forgettable, terribly paced, had crammed plot, and bad animation in comparison. A far cry from the first season... I watched it once because I love the first season and manga so much, but never watched a second of it again for some reason.