Utah plays their best game against BYU... LOL, what a pathetic delusion that is.

2010: Every win Utah had (they had 10 of them) was a wider margin of victory than the BYU win. I mean, Utah scored 60+ on Iowa State that year.
2011: Probably safe to say Utah's best game of the year this year was in fact against BYU, but you could argue that much of that score was due to silly mistakes by BYU, not necessarily Utah's dominance. I might suggest that the UCLA game (Utah won 31-6) was in fact their best performance of the year.
2012: Utah beat Cal and Washington State by a combined 98-33. No way was the BYU game the best game of the year for Utah in terms of performance.
2013: You could again argue that we played our best game against BYU, but Utah beat Stanford by a touchdown as well that year, and they are a better team than BYU, so I'd probably give the nod to that game. Though Utah did have zero turnovers against BYU that year.
2015: LOL no... not close. We beat Oregon 62-20 that year.
2016: Lots of contenders for best game played this year, BYU is not even close to one of them. Beat ASU by a ton that year.
2017: Beat UCLA 48-17
2018: We had many wins that year, only three by less than 10 points, BYU was one of those three. Not close again.
2019: Beat Oregon state 52-7

You could argue the opposite is true