I've seen just about all of Eric Staal's pro games, and he's never had a bad one. He's been invisible a few times, but he's never had a game where you went "wow, that kid sure played bad tonight". I've been very impressed with his progress and rapid acclimation to the NHL game. He's probably the rookie this year that i'd say has the most upside growth potential.
As for the others, i've been really impressed with Matt Stajan. However, i'm not sure how much growth potential he has. He plays a really fantastic energy type game like a Kevyn Adams, but i'm still out on if he can put up the scoring numbers to match his effort. It's not a knock though, Kevyn is a great energy player.
Zherdev looks great with those slick moves. However, he needs to get his focus on defense or else he's going to be Chistov next season. Flashy player, just needs to work on the fundamentals a little more.
Pitkanen has fantastic upside as well. He's probably as close as we're going to get to the next generation Rob Blake. I'd say he's going to be a perenial all-star.