Barnett is the biggest in terms of physical size, and the most well rounded (don't forget wrestling when trying to fire back saying Nog is more well rounded) of the field. Statistically he should win.
Vanderlei has better tournament experience than the other 3 (except maybe Nog, he's been in a few in PRIDE and RINGS) and the speed.
CroCop obviously has the striking advantage against anyone. Vulnrable with submissions, but hard as shit to be taken down.
Nogueira is under Barnett the most well rounded. More dangerous with submission, more dangerous with striking than Barnett, but his wrestling isn't there like Barnett's. But it don't matter because against anyone except CroCop he's gonna be able to do fine on his feet. On the floor, against anyone period he'll be fine, if not at an advantage.
Knowing all this, I gotta go with Nog.