Don't fear Raider Nation; I'm not including Darrius Heyward-Bey in my list. Regardless of how pitiful his first season was, you can't go bust on a guy after only one season. However, I do feel as though he epitomizes much of what can go wrong with picking the position.
Teams get enamored with a player's height, speed, or other standout trait(s) without taking into account the most important observation of all: Can the guy play football?
The criteria is fairly simple.
A receiver needed to have been drafted in or since the year 2000 as a first round draftee, and never had more than one impact season. As far as impact seasons go, I would consider more than 1,000 yards receiving or double digit touchdowns to be expected of first round picks.
Rather than singling one guy out as the worst, I'll do them in chronological order and let you make the final call. I understand a couple are still playing and thus could potentially rewrite their careers, but to this point they've given us no reason to think they will.
*I'd like to thank Wikipedia for some tidbits of information, specifically regarding Sylvester Morris, R. Jay Soward, and Freddie Mitchell. The rest of the players are recent enough that their stories are still fresh in my mind!