Community is one of my favorite sitcoms. Seasons 1 - 3 were the best part of the show. Dan Harmon's absence in season 4 and the absence of key characters in the later seasons ruined the show. If it wasn't for that I would have given it a 5/5.
Season 1 is grounded and normal. Season 2 has high-concept episodes while season 3 has surreal elements and darker stuff. The shift in tone over the three seasons is brilliant.
Seasonal ranking: 3, 2, 1, 6, 5, 4
The characters are also part of what makes the show awesome and they each offer a different kind of humour - Jeff has witty humour, Abed has meta humour, Troy has physical comedy, Pierce offers offensive humour, Chang is crazy etc.