If one is on the shoulder of the last defender and about to be put through by Silva or Nasri with the keeper to beat, I'd say Aguero. His timing and pace makes it odds-on a goal.
If one is on the edge of the box with 2 or 3 meaty centre-halves and/or tough-tackling midfielders barring his way that need wriggling past, I'd say Tevez. He has scored many goals "out of nothing" like this to break the deadlock.
But as a combination - amazing. No wonder we score at least 2 or 3 every game.