Smith is the best fit for the team's needs - his natural position is PF, which is our weakest starting spot (once Avery comes back). He's one of the league's most dangerous transition threats at the 4 (a great fit with Rondo). And he's young with a clean bill of health.
Varejao's a close second (I dock him for injury concerns).
Big Al would be nice, but he's not a great fit for our needs.
Any of these trade scenarios depend on Jeff Green busting out and souping up his trade value. Atlanta and Cleveland in particular are starved for talent at the 3-spot. If Green starts putting together stretches of games like last night in Utah, those teams might be partial to building a deal around him and smaller assets (Lee, Sully, Melo, future picks).