It probably has to be De Niro and Scorsese. So much high quality stuff : Mean Streets, Taxi Driver, The King of Comedy, Cape Fear, etc.
Hitchcock and Cary Grant are close runners-up. Notorious, North By Northwest, Suspicion, etc.
I'd put Hitchcock and James Stewart in third place. Rear Window being their finest achievement in my opinion.
I think that the list needs to add David Lean and Alec Guinness as well. So many films : Great Expectations, Oliver Twist, Bridge on the River Kwai, Lawrence of Arabia, Doctor Zhivago and even A Passage to India(even though I think that Guinness was miscast in that).
Quick Update :
I'd also add Howard Hawks and Cary Grant for things like His Girl Friday, Bringing Up Baby, Only Angels Have Wings and Monkey Business.