This is an interesting question. I look at it a couple of different ways, depending on how you set the criteria.
If you mean which recruits never panned out given the incoming hype, then Paul Harris is a pretty good choice. He was a solid, three year contributor who put up decent numbers and actually had a very, very good sophomore year playing most of his minutes at 2G, but given that he was one of the top incoming recruits that year and hyped as potentially being able to provide a little of what Carmelo brought to the program, he didn't live up to that. Other candidates in this category for me include: Mike Brown, Tony Harris, LeRon Ellis, Chuck Gelatt, Craig Forth.
If you're talking about players who transferred after not making an impact at SU but went on to have solid careers elsewhere, then my list changes: Earl Duncan, Keith Hughes, Bobby Lazor, Richard Manning, Rock Lloyd. All of those guys are players who COULD and probably WOULD have helped the program if they'd stayed.
If the criteria is players who came in with a good opportunity but failed to actualize the potential value they brought to the program [sometimes submarining what otherwise might have been promising squads with their negative presence], then my list would be: Michael Edwards, Josh Wright, James Thues [the one guy on this list who wasn't a negative presence], Dayshawn Wright, Louie McCroskey.
If we're talking about kids who were near misses, then Winfred Walton tops that list, given the impact I imagine he would have had at SU.