I have a very unpopular opinion, Seasons 3 and 4 are better than the first season and just as good as season 2. The Art-style in Season 4 was fantastic, though Season 3's was mediocre.
Favourite Episodes:
Season 3: Dos Boot
Season 4: The Scrying Games
Least Favourite
Episodes:
Season 3: A Third Dad Cartoon
Season 4: Chicken Scratch/Beau Tie