For myself seasons 4 is easily the best followed by season 2. Season 3 takes awhile to start going, but when it does it does great, of course then the writers strike kills it right at the height of its material.
Seasons five suffers after a strong start then focusing two much time on softer (or out and out humor) episodes, that just really don't feel appropriate for that season. But it does produce some great episodes.
Season 6 I still haven't found a utterly great episode, but I have only disliked one of the episodes this season. Its probably the most consistent season, but it isn't the best.
I honestly have a hard time deciding out of seasons 3, 5, and 6 which I would rate better.
And then there is season 1, my least liked season. It took the show a while to figure out the characters (and the actors to get their roles down), the show often used very poor actors, and played far less seriously with the material. That does end, but its evident through most of the season. It features more episodes I actively dislike then any other season, and really has only one great episode.
And I love the show, but its primarily due to the relationship between the two leads. Its the strongest brother relationship I have ever watched on tv, and one of the things they almost always get right, even in crappy episodes.