I've killed elk with the following rifles, and probably a few others I don't recall offhand:
270 Win, Win 70 Featherweight, 140 grain TSX and 150 grain Nolser PT
308 Win, Win 70 Featherweight, 150 grain Nosler partition
308 Win, Ruger 77L, 150 grain Nosler Partition
30.06, Win 70 Featherweight, 180 grain Nosler Partition, 168 grain TTSX
30.06, Ruger #1, 180 grain Nosler
30.06 Kimber 84L, 168 grain TTSX
300 Win Mag, Win 70 Supergrade, 180 grain TSX
300 Win Mag, Kimber 8400, 180 grain TSX
300 WSM, Kimber 8400 MT, 180 grain TSX, 150 grain TTSX
375 H+H, Win 70 alaskan, 300 grain TSX
I've killed more elk with the 30.06 than any other caliber, but that is quickly being challenged by the 300 win mag. Currently the Kimber 8400 in 300 win is what i consider my "go to" elk rifle, but still use the Kimber 84L on at least one elk each year. I'm pretty comfortable with any of the 30 cals, but feel the .308 is a bit light when hunting BIG elk. It is fine on cows and spikes. Stay away from the extrmes on both ends, heavy and light, and you will be well served throughout your hunting career. I've killed elk out to 500 yards with one shot from a 30.06 mulitple times, but the 300 Win mag makes range estimation a bit less critical. They aren't anymore dead, however. Unless you have made a particularly poor choice, it isn't the caliber that matters, as bullet placement is king.
llp