Arsenal is playing the best football in the league for sure, but they've been lucky to acoid any injuries on key players. In my opinion they lack depth in the defensive line and an injury or two there could hurt them a lot. Also, I think their game will suffer during the winter when the pitches get bad. Add to that the fact that many players are young and/or lack experience in the premiership and I don't think they will manage to be consistent through the whole season.
Chelsea is looking up now but they will suffer from the lack of Mourinho and the fact that the new manager is trying out a new system. Also, they are incredibly reliant on Drogba. If he leaves or gets injured, they're screwed (and they will miss him during the African Cup).
ManU has already suffered a lot of injuries and are still second in the lague. Their more direct game style is better suited to the harsh winter conditions and their squad has a good mix of youth and experience with a very strong manager. Although they rely heavily on Ronaldo for their offensive game they still have more options than Chelsea, with Rooney, Scholes, Tevez, Nani and Giggs all being capable of deciding matches. They definetely have the best squad depth and should be able to handle any injuries except from Edwin.
But to be nice... I'll let you go for $80
Ps. The offer stands until Nov 3, 12:45 GMT.