I wouldn't mind United or Porto. I think we could beat them both. Ajax worry me - they are young, hungry and playing without fear...they remind me of us a few years ago. In our current state though, drawing Porto or Ajax could well lead to us showing the 'arrogant complacency' Poch has highlighted recently and doing ourselves over as we did in our first CL campaign and then applied the icing on the cake against Gent later that year.
I think we can take Juventus. They really aren't that special. City and Barca are the only teams who genuinely scare me.