Aguero would be the best IMO, since a) he should work well with Messi and b) he can play out wide and centrally. Play him on the wings with Messi at false 9 in most games, but switch to Aguero at CF and Messi at RW as a plan B when things aren't working out. He's gonna be pretty hard to get though.
Suarez would also be good, he's definitely been better than Aguero this season. However, I think Aguero would be better than him for us, since, like I said, he works well with Messi. Though from what I've heard, Suarez can also score from the wings. He'll probaby be easier to get than Aguero too. However, from what I've heard and from looking at his shots per goal stats, it seems to me that he's a bad finisher, which could be a problem.
Neymar would be a risk. May or may not pay off. Could be awesome if he does succeed though.
Can Cavani and Falcao play out wide? From what I've heard, they can't. IMO, what we need is someone like Villa, Suarez or Aguero, a striker who can play out wide. Not someone like Cavani, Falcao or Ibra. That way, we can use Messi in his best position in most games while having another goal threat, and move Messi to RW and use that striker in the center as a plan B whenever things just aren't working out.
Lewandowski could be a good option as well, especially if he can play out wide. Not to mention that since he wants to leave, he'd be a bargain.