It's almost certain that Chelsea and Barcelona are through to the semis, and both are strong enough to go on and win the competition.

I can't see either Villareal or Porto going on to win it if they get through to the semis though.

But if United and/or Arsenal go through, either one of them could go on and win it.

So I think the winner will come from one of those four; Chelsea, Barcelona, United, Arsenal.