Man, it's all in how you define this. To me, Jada and Luda are the two biggest wastes.
Kiss doesn't have that clear cut classic. The Last Kiss is his closest, but with the quotables and great verses he's dropped over the years, he should have a classic album (plus, he's only put out three albums in 15 years). I thought he dropped the ball in '04 after he put out the Champ Is Here mixtape. Coulda been KONY if Kiss of Death was a better album (wasn't trash, but it was a step back from the hype his mixtape produced).
Luda is technically a near flawless rapper. Perfect voice, flow, presence, etc. He's been on tracks and hung with or destroyed most of your favorite rappers. He always stands out, but he's another guy that should have had a bigger impact. I know he sold records, but something's missing.
Honorable mention: Grafh. For the life of me, I still don't know how he didn't blow up in the early 2000s. Wildly talented and had some great mixtapes.