IMO any bullet in the 30 caliber class will take an elk... But the important part is that you learn what area you will be hunting and what the shot distances could be... Then pick your rifle according to that...
I personally would go with a .308 but that is from doing a little research about a great caliber that is good with bullet weight and energy out to long distances... I mean the FBI, most Police Department Sniper units, and US Special Forces Snipers use the .308 for their craft... That says a little something about the caliber
Another round that would be good and is also a 30 caliber round would be the classic 30-06... You can go almost anywhere in the world and find a box of ammo for your gun and it performs almost identical to the .308 from what I have read. The 06 also offers a large selection of bullet weights so no matter what game you chase it has a load to fit. Plus its what millions of GIs used to ensure our freedoms!