I would've said it's RES by a landslide. I'm a long time BYU season ticket holder. I've also been to many games at RES and they have a really good atmosphere up there.
Fast forward to now and the last 2-3 seasons BYU has figured it out. They've done a great job at turning the game not just into a game, but an event not to be missed. It's MUCH better than it used to be. The atmosphere really is night and day what it used to be. The gameday atmosphere at BYU is fantastic now and on par if not better than RES and there's no way I could've said that some years back.
For those of you who haven't been to a game at LES in several years I urge you to go. Especially night games. LES under the lights absolutely hits different.