Key stuff is all pretty good IMHO, but judging solely by anime, I'd say Clannad After Story > Clannad > Air > Kanon > Angel Beats > Little Busters.
Clannad, Air and Kanon are all in the same league, while Angel Beats and Little Busters are a tier down for sure.
The forced humor was a big turn-off in both Angel Beats and Little Busters, and actually was so bad in the latter that I couldn't even finish watching it. Angel Beats was at least worthwhile when it wasn't being "silly," but Little Busters was just... very unappealing, and is the only Key anime I haven't enjoyed thus far.
Kanon and Clannad are very, very similar, even reusing a lot of the same ideas, but Clannad feels like the superior take on the formula in every possible way (to me). Kanon is still very good, though, and at least far less depressing, so if you're susceptible to emotional devastation, it might be the safer choice.
And Air is far MORE depressing than any other Key work IMHO, but in a really surreal way. Air is really cool, and is the anime that got me into Key works in the first place. It feels very artsy and unique, and stands out among the other shows quite a bit. Not as engaging as Clannad due to less fleshed-out characters (which I'm sure the visual novel remedies), but it does feel more "sophisticated" in many ways.
This is all just my opinion, though, and I suspect it's a lot like trying to rank Final Fantasy games (or, to a lesser extent, Star Trek movies) -- nobody else will ever fully agree with your list, and everybody will always have their own unique take on the matter that they'll defend to the ends of the earth.
-Tom