It depends on how you define it.
If you're talking about the ability to break away and outrun players, I think Dorsett would be about as high on the list as one could get.
However, if you're just talking about being slippery -- I don't think anyone in the draft holds a candle to Justin Hardy. He doesn't have ridiculous speed. But he has great acceleration in the first steps. And when you watch his tape -- it is literally mind blowing that he can get virtually always open on any route. He has some 10+ reception games where with a better QB, he could have 15+. His ability to run routes, cut on a dime and shake press coverage with feet and hands is exceptional. He's like jamming a greased pig.
He's probably not a fit for Seattle. He doesn't have great size or speed. But it's as if he's made out of Teflon. Teams slant their coverage to him, or play high/low and he just splits them effortlessly. It's actually impressive to see just how comfortable he is catching balls in tight windows. He is sublimely talented at shaking defenders and breaking free when the ball arrives. And his quick burst/first step allows him to make a move and shake defenders for extra 4-5 yards even when it looks like they should drop him immediately after the catch.
He should make a terrific slot/flanker receiver in the NFL. Much like Baldwin is for us. Very reliable -- able to play inside or outside. Off LOS or beating press at SE.
For big WRs, I like Breshaud Perriman. Watching him, it's really remarkable how open he does get. Even when the balls don't come his way. He has a real knack for getting open. Just a very difficult player to cover.