Of course, this type of question is always subjective, but for me this is the epitome of a group Hip Hop song -showcasing the individual talents whilst at the same time having the cohesion to rival any individual effort...
2) Three 6 Mafia (before the mass exodus that left it to just 2-3 people)
Wu-Tang Clan and Three 6 Mafia at it's peak embody everything a group is supposed to be, pretty much like firepower said. not only did the specific group members bring it, but they're affiliates (Gangsta Blac, Streetlife, M-Child, Killarmy, The Kaze, etc) were dope as well, and there was very little dropoff when those members would pop up on tracks.
1) UGK & Mobb Deep.
Have to put those two together cause even though I'm from Texas, I heard of Mobb Deep before UGK. But UGK put out nearly flawless projects every time out, and very good solo projects. Mobb Deep have duds here and there.