I know it's recent, and so easy picking, but I've gotta go with Curry.
I still remember sitting in a Saturday class, tracking the draft of my phone, and praying we didn't blow the #4 (FOUR!) on him. When his name popped up, I turned my phone off, slumped down in my seat, and missed an entire lecture, I was so pissed off and depressed.
That pick pretty much got Ruskell fired, and certainly didn't help Mora's case to stick around. He was appraised as a can't-fail pick and bombed so spectacularly, it really punctuated the end of era. We've whiffed on a handful of players, like any team, but the fallout from Curry was so radioactive, it has to make him number one of our all-time busts.