So, at half time (over two legs), I'd say it's advantage Arsenal and Porto.
I'm not underestimating Villarreal, but we're going to be at home with an away goal in the bag. Villarreal are not big goalscorers either - they have been drawing a lot, or winning 1-0. We'll come at them in the first half like they came at us. The difference will be that we won't wilt in the second half.
It's hard to call the Man U / Porto second leg. Man U can beat anyone on their day, but their form has been patchy recently. Porto's home support will be intimidating. It should be an epic clash. I'm going for Porto (mainly because I hate Man U).
Arsenal Vs. Porto with Arsenal going through (obviously).