f2.8
i think the tamron is considered by many to be only second to the 70-200f2.8 IS II...i think the sigma, and tamron are both ahead of the original 70-200mmf2.8IS...
Except not every place (nor everybody) agrees. I'll use The-Digital-Picture, which I know is only using one lens and there can be sample variations and testing error.
As far as the Sigma OS version, the MKI looks sharper across the entire frame at most FLs. At 70 they seem same/close in the center, and at mid frame (Sigma slightly better), and Canon is better at the edge - the sigma has the slight edge. From 85mm on the Canon seems better. But Sigma has newer technology so OS should be better.
As far as the Tamron VC version, it looks sharper wide but worse at the tele end compared to the MKI across the entire frame (so slight advantage Tamron). Again it has newer technology so VC should be better.
My guess is the AF speed goes to the Canons. That is only a guess, based on different Sigmas and Tamrons I have owned. These Sigma and Tameron lenses are newer technology and so I could be wrong.
So based on that site the MKI handles sharpness pretty darn close to the newer lenses (except maybe the MKII).
I picked the Canon (MKI) over the Sigma for a number of different reasons. The Tamron was not out when I got mine, or I might have went that direction (if I could have picked it up for the same price).
Again I think all Ls and both the Sigma and Tamron are sharp and excellent lenses, so pick your poison.
