I believe Stewart was a 3rd round draft pick, so regardless what he did in pros (and he was decent for a while), he should not be named among all-time flops.
Leaf is for sure among all-time busts, but already mentioned.
My choice would be Heath Shuler...right around Raj territory.
Honorable mention:
Tim Couch & Akili Smith & Cade McNown
All 3 of these jokes taken 1, 3, & 12th slots 1st round of 1999 draft.
:WTF: