You mean "season", right?

If you are talking about Season, then I definitely say season 3! In my opinion, Season 1 was much more like a kiddy cartoon, like it was definitely directed at children to watch.
Then by season 2 we can notice that the jokes are directed both and kids AND older people, so more teenagers can enjoy it. Also season 2 started to get more heavy on Story, which is a good thing! Season 3 was just EPIC! Some of the best fights and plot of the whole show!! The last couple episodes were amazing and, in my opinion, not really directed at young children anymore!! For example, D-Stabilized seemed a bit too graphic for kids! But it was intense!! I (being not a little kid) LOVED it!

Sorry, ranting!!

Most of my favorite episodes come from this season :)