I'd be happy with any of them. Halladay would probably be best, since he'd give them the ability to have 3 ace-quality pitchers on the staff. Even with a mediocre offense, I don't see how you beat the Sox in 7 games if you have to face Halladay, Beckett, and Lester twice each.
Gonzalez would probably be my second choice, since he'd provide that elite power bat; however, Martinez wouldn't be bad either. His ability to play catcher and first base would give the Sox a ton of flexibility. While he may not have the pop of Gonzalez, his ability to provide better offense than Varitek (and certainly Kottaras) would be an upgrade for the Sox in the games he played at catcher.