I won't claim it to be the best, but my personal elk rifle is a 7mm RemMag. My backup is a .270. Personally, I am completely satisfied with the performance of both, and have no intention of changing. If I were to go and buy a new elk rifle though, I would seriously consider a .30-06. Mainly, that would be because my Springfield 03 is in that caliber, and I could consolidate my calibers a little bit, but also because it has been proven time and time again to be an excellent and versatile cartridge.
It's a tough question to ask though, because there are a few answers, most based on personal opinion, and frankly, most being correct. If one guy tells you a 300 WinMag is best, and another says a .338 is best, both are right. It really just boils down to what you care for the best. When I got my elk rifle in high school, my dad gave me a choice between three rifles/calibers: 7mmRemMag, 300 H&H, and I think a .300 WinMag. Of the three, I just liked the 7mm the best, and so thats what I chose. It worked out, because I kilt me a wapiti with it shortly after.