Season 1's my least favourite as well. I know it was what started SpongeBob in the first place, and it does have a scant few highlights, but I hate it because of animation quality.
Season 2 started out on a great note, much better animation quality, but only half the season got a 10 out of 10 plus for me.
Season 3 is like a typical heads and tails coin. As the nursery rhyme says, when the episodes are good, they are very very good. When they're so so, they're so so. When they're bad, they're mediocre.
But so far, I think Season 4 is my favourite. Don't yell at me for saying this, but I just, all the episodes on it, great. Fab great. There have only been a few twinkles of or unseen episodes, and those I have seen range from 9 out of 10s to 12 out of 10s. As a matter of fact, I don't think I've seen so many 10-12 out of 10s in a single season since Season 7 of The Simpsons. A classic season. Can't get enough.