Geez, ask me to pick a favorite child why don't you...
I own two out of these three - I don't have a P30. I might get one while they've got this rebate going. Its trigger has never really grown on me, but neither has the USP trigger and I shoot those pretty well, which makes me question why we complain so much about triggers, but anyway. The P30 ergonomics are obviously an advance on the USP, but I'm used to those bricks and finger grooves aren't for everyone.
Now the VP9 was my intro to striker-fired pistols and I'm still not a huge fan of the class (even though I've since added a Glock 17 to the armory - I know, I know) but I do like my VP9. Really, though, no H&K pistol is a bad choice.