I might as well step into things here.
Dion Phaneuf is by far the best Defenseman prospect in the world right now. A number of reasons, in my opinion, make this the case.
First and probably foremost is his attitude. As anyone who was watching the post-Gold medal coverage saw, everyone talked about an even-keel and that's exactly the approach the Dion takes to the game. He never gets too up or too down and that gives him tremendous focus. This pays off for him on the ice as well because he takes his time and never rushes - even if he's pressured, he makes the best choice and follows through with it.
Staying along the lines of attitude, Dion would be my first choice for captain on any team. He leads by example and is such a hockey savvy kid, it's truly incredible to hear the things that come out of his mouth. He always puts things in the proper perspective and when you're a player on the bench seeing Phaneuf kill a 2 minute, 5 on 3 penalty, in its entirety, that is a huge motivational boost.
His stamina and endurance is second to none in the game of hockey and he's one of those rare guys (as Pierre Maguire mentioned) that just gets stronger as the game goes along. In that sense he's a lot like Nick Lidstrom, he can play 35-40 minutes in a game and not be effected.
Another one of Dion's strengths is his awareness and checking ability. You just can't beat the guy. Ovechkin, Malkin, Olesz...all these top prospects could not make a move. For a 19 year-old kid that's very impressive.
Everybody knows about his hitting and his knack for delivering numerous crunchers - no need to talk about that.
Lastly, his puck skills should not be over-looked, while it's his defensive side that most people covet; he makes an excellent first pass, has the ability to skate the puck out (and even into the opposition zone) and he owns quite a heavy shot from the backend on the powerplay.
Phaneuf is the real deal and in my opinion is the next Hybrid Defenseman, a combination of Scott Stevens, Nick Lidstrom and Chris Pronger. I'm not knocking Suter but he isn't nearly on the same level as Phaneuf.
For my money, I'd take Phaneuf over Ovechkin, Malkin, Lehtonen or Fleury anyday. I think he's probably not only the best defensive prospect in the world, but also the best prospect in the world, period. He's as close as you get to a sure thing.