So many games to choose from for a variety of reasons.
28-21 vs Miami. (Detmer)
vs Penn State (Detmer w/ 576 yards against 4th ranked defense)
52-52 vs SDSU (Detmer IIRC)
BYU vs TCU -the turning point. BYU really needed to gain some confidence after years of falling short. (Beck IIRC)
BYU vs Texas - Taysom was awesome - simply unstoppable. Any game that results in the opposition getting fired is pretty solid.
BYU vs Utah - 1983 - Steve Young. 55-7 Not all that significant of a game per se, but it was to me.