I think if you just pick one attribute and go with that, you're spelling disaster as a GM. Obviously, you have to have speed, but if you can't catch the ball its' no good. If you can catch the ball, but can't run the routes, that's no good. If you're elusive (YACs), but don't catch the ball or have the speed to get off the line. The attribute you forgot to mention is the intangibles.
Wes Welker is a good example of not having all the great physical attributes, but still having the ability to be semi-successful even in awful conditions here. Those are things that you can't always see in college films and visits leading up to draft day, and if you think you can see some of them, it's just a feeling, nothing tangible you can put on a guy.
It's just too important NOT to judge a prospect on just one attribute or you end up with fast guys that can't get open or run routes or in other ways play for your team...