What is going to be the best round for deer?
The definitive best does not exist. The best round for
you can only be determined by
you.
Do you think you will ever hunt anything larger or smaller than deer? if so, do you want one do-it-all rifle or will you specialize by purchasing multiple calibers for each hunt?
Do you like the big boomers or the more mild mannered cartridges?
Do you prefer a short action or a long action?
Are you of the mindset of a larger diameter bullet at moderate velocity is better than a light bullet at high velocity or the other way around?
Do you hunt thick, wooded forest / timber or wide open spaces?
Do you have any physical limitations that might restrict the gun or caliber you choose?
Would ammo availability play a part in choosing a caliber?
For me, I hunt with the 6.5x55 and love it. Great accuracy, very lethal, good retained energy at longer ranges, mild recoil. Ammo can be found a the big chains, but not likely at Wal-Mart or most rural gun stores should I forget and leave my ammo at home. I'm willing to live with that, many people like the flexibility of a common caliber.
The .308 is a fine choice. As is the .270, the .243, the 30-30, the .30-06, the 7mm Remington Mag, the 300 Win. Mag, the 25-06, the 7mm-08, the .260 Remington... I think you get the idea.
Bottom line: Get the caliber you are most attracted to, for whatever reasons you have, and go for it. Deer are not armor plated or particularly hard to kill.