It's Aaron Maybin and it's not even close. Don't know how/why anyone would call Spiller their worst first round pick ever. The dude didn't match the hype but he was productive and had an excellent year in 2012. Maybin made a handful of tackles, zero sacks and was cut after two seasons. With his size and athletic ability he should've been converted to tight end. 6'4", 250, ran a 4.64 40, had a 40" vertical, a 10'10" broad jump, 22 reps on the bench press. Jauron went gaga for his burst off the snap but that's all he had in his arsenal. Couldn't stack and shed blocks, had no counter moves, pure speed rusher.
After that, I'd say Mike Williams, but he was the number one OL on every teams board that year and was one of those "sure thing" types that never made it.
Losman, Whitner, McKelvin, etc. none of those guys ever lived up to their draft slot but they at least had stretches of time where they contributed. Maybin contributed zero, which is why I can't understand how he isn't the unanimous choice for worst first rounder ever selected by the Bills, haha.