Seasons 1 and 2. I love how the world/universe was gradually revealed, where a character, alien species or technology could be invented to serve a joke.
I love the others too, but it got to a point where the universe was established, and it became more about exploring stories within this setting rather than world building.
This is a hard one. So many good episodes in seasons 1-4. If I had to pick one I would have to go with season 2. Only sub-par episode in that season imo is The Honking.