90mm, recoilless (don't want to be flinching). HEAT for those tough quartering shots.
Elk are not invincible. I've used .243, .30-06, .300 Wby, and .375 H&H. I'm perfectly happy chasing elk with the .30-06 (180 grain JACKETED bullets). If I'm hunting more open terrain, I'll prefer the .300.