The most athletic, but was hampered by lack of knowledge, was likely Ziggy.

It also gets hard comparing between positions: Mo Elewonibi and Bart Oates were great offensive linemen -- and it can be debated if either of them were actually the best, Jason Buck as a linebacker, Lee Johnson as a kicker, Chad Lewis at Tight End, and likely Austin Collie as receiver. Though you keep running into issues with what "best" is; most athletic, most skilled, most valuable to the team, most success at BYU, most success professionally, etc.