Kobe, Nash, and KG are all amazing players, but if I was truly building a franchise I'd want to start young.
Dwight Howard- The man is an absolute monster. Need I say more?
D-Wade- Apart from the fact he's a brilliant player, he has a fantastic attitude; in a Jordan-esque way, he doesn't ever give up. Also, we know he can either carry a team (when shaq was out for the first half of this season) or co-exist with other superstars (see 2006 playoffs).
The 3rd pick is a bit harder, I think:
Dirk- He can be boring to watch, but who can actually guard him?
CP3- Assuming the stress fractures heal. He's one hell of a playmaker. If he was too risky, I'd take Deron for sure. He'll be the next Kidd in 3 seasons, max.
Durant- What can't he do?
Oh, wait. He's dominated in college, but hasn't actually played in the NBA yet. It could either pay off big (think Melo) or be a nasty surprise (think Laettner). With his crazy arsenal of moves though, it's a risk I'd be willing to take.
LeBron- When he plays like he is now, not when he's in selfish, sulking mode. Could he really exist on a team where he wasn't the only star though?
Melo is a notable omission, mainly due to that gutless bitch-slap he delivered in December.
If I could only pick 1, it would be Wade.