I'd select Calvin Johnson without much hesitation.
The problem with the OL isn't so much that the Bills haven't invested first round picks up front trying to find a Pro Bowl LT. Instead, its that they haven't bothered investing a few 2nd-4th round picks over the years in order to bring some good, but maybe not great talent at the other OL positions.
I still believe the interior of the OL is our real weakness offensively and it all stems from the fact that they struggle to run block. Yes, I said run block. That's why McGahee struggles to average 4 yards per carry. Its tough to run when your guards and center are being knocked three yards into the backfield every other play. Losman, in turn, struggles because without a reliable running game, he's constantly facing 2nd/3rd and long situations. That forces more 5 and 7 step drops and longer developing plays, which again, puts the weak OL right back at center stage.
So yeah, adding a LT would help a little, but until the Bills decide to bring in some quality players between the OT positions, its probably still going to be a mediocre OL.