My list is pretty much the same as most posted, but my "could've been a contender" pick is Losman.
Tons of talent, but a bit immature and frankly weird when he came here. Unfortunately, the coaching and the culture was poor, and instead of competent mentors with a plan it was the last disasterous year of Mike Mularkey, a guy who quit his job at the end of the year, and Dick Jauron, who is a pretty decent defensive coordinator who has no business running an entire team.
He was completing 62-63% of his passes over the 2 years when he saw real playing time. For a guy who was known more for long passes to Lee Evans than much of anything else, he did make some decent throws in the short and intermediate game as well.
He's my pick for "might've been good" had he been on a competent team with real leadership. Having his leg broken in training camp where there were often fights and generally out of control players doing their own thing pretty much speaks for itself. Coaching was putrid, and the organization was such a mess from the GM down.