These are all really good suggestions. It's hard to argue with the statistics, and I think it's a safe statment to say that the .30-06 has killed more deer than any other round. That's mainly due to it's longevity and ready availability.
I'm a .270 guy myself and love the flat-shooting trajectories of that cartridge. But elk are about as heavy of game as I'd feel comfortable going after with it. If you are after things bigger than that, the .300 Win Mag would also be excellent. I also like the 7mm that Ghost and Pharmer suggested. Great cartridge.
Unless you're going to buy something that is totally obscure, I wouldn't worry about caliber availability. Everything posted on this thread so far can be had at any good sporting good store, and with the internet these days, you can get a supply of any cartridge with ease.
Whatever you end up with, take JNRBronc's advice to heart: As with anything, shot placement is the key with a high-power. Shoot it with the same care and precision that you do a bow, and it will be an effective tool in your hands.
"Beware of the man with only one rifle; he probably knows how to use it."