i would take dirk over wade and lebron because i think he is a more unique player than they are. he is the kind of guy that causes all kinds of defensive problems for teams and makes scoring easier for all his team mates.
Again ... not to take anything away from Dirk (I love the big German!), but Wade and James cause just as many defensive problems (if not more), and they both set up their teammates well. Especially LeBron. That guy's just a freak. He's 6'8", over 240 lbs., and he's faster up and down the court than just about anyone in the league. Add the fact that he's one of the top three passers out there, and I know I wouldn't want to try and stop him defensively.
Dirk is a tough cover though. If you put a four or five man on him, he easily blows right by them. Put a three on him, and he just shoots over them. If he was a better passer, there would be no way to stop him.