My vote goes to Taysom's knee injury. If he is healthy, even if Riley does still play in all those games, he gets reps, he gets experience, he gets an entire off-season to prepare for last year. Less time rehabing and more time learning and practicing and throwing. Mostly throwing. Absent this play I truly believe BYU win's against UVA and Utah last year. 3-0 with a win against the 16th ranked team and BYU is 10-15 by the time they roll into Madison and South Bend. Better chance to win one of those means BYU may have been ranked to end the season.
No other play comes close.

However...

Riley pick six against Boise? one game impact at the least, a few weeks of BYU getting votes in polls at the most.

BB fumble? One game impact on the team, one off-season of bragging rights that ends ignominously the next September anyway. No real effect on the team.

Riley missing Hoffman? This is probably the closest as it would have given BYU a huge boost in polls, morale, prestige, etc. and changed the face of college football that year. Also would have been a HUGE win to strut out to recruits.

Heaps fumble? BYU loses 47-10. Whoop de freakin' do!

Riley's broken back? This could be the dark horse. If he doesn't break his back, Taysom never plays against USU....hmmm.

Just. Win. Baby.