Casillas
Lahm - Vidic - Kompany - Marcelo
Schweinstieger - Banega
Nani- Silva - Ronaldo
Drogba
Casillas in goal because he's the best shot-stopper in the world, which is what you need. His weakness in the air isn't exposed vs Barca.
The defence I'm completely unsure about. I put Vidic-Kompany simply because I can't think of anyone better, but in a perfect world you should definitely have a Rio-like player there, maybe even two. Can't think of anyone good enough though (Thiago Silva sounds like he could take that role, but I've barely seen him play and never taken much notice of him when I have). Same with the fullbacks - I don't rate Lahm or Marcelo particularly highly, but can't think of anyone else better at the moment.
I chose Banega as he has given Barcelona a lot of trouble a few times now, he seems to enjoy playing against them. And Schweinsteiger next to him should bring some pace and drive through the midfield, while also being very good at passing the ball himself.
It was a hard choice between Silva and Ozil in that role linking the attack and defence. I went with Silva simply because I think he's in better form at this moment in time.
Ronaldo hasn't had the best of times against Barca, but...well, it's Ronaldo. Can't leave him out. I've gone with Nani on the other side as he's simply the next best winger in the world, and I want a proper winger there.
And upfront I wanted someone big and powerful enough to give yourself the outball if you need it, but also skilful enough to combine well with the other attackers. Drogba isn't in the best of form, but I'd bet on him pulling out a big one for this match.
And if you want three subs, I'll give you RVP, Rooney and Toure.