I think .308 is about the best deer caliber out there? But like others have said there are many really good calibers. I also think there are some older calibers that are as good or better than the .308 but there isn't much of a bullet selection for those and it's getting tough to find a rifle in those calibers (7x57 and .303 British).
There are also situations that could make a different caliber better than the .308. A .270 could be a little better if you're shooting long distances. A 30-30 is great under 100 yards. A 30-06 is right there with a .308 but lets you shoot a little heaver lead if need be. If you don'i like recoil then a .243 would be the way to go. 7mm-08 is also a really find caliber.
There are many great calibers, I just think the .308 is about the best all around general purpose caliber today.