.270 Winchester
.270 Winchester is my #1 pick, but .243 Win., .257 Roberts, .25/06 Rem., .260 Rem., 7mm-08, .308 Win. 30/06 Sprg. are ALL suitable and highly effective Deer Cartridges.
I personally tend to stay away from most Magnum, Short Magnum or Super Short Magnum because of the "Barrel Burner" aspect, as I've heard quite a bit on that (most hunters never burn barrels, though), along with the extra recoil, and cost.
Always consider the fact that You're hunting WhiteTail and Mule Deer (not widebeest, buffalo and lion), and unless You're making very long shots most of the time, any of the above Cartridges are fantastic.
I agree with a previous poster, a 3x-9x scope is sufficient also.
I prefer .270 Win over 30/06 Sprg. The 30/06 has ALOT over the .270... .308" bullets are more available in more Brands/Styles/Weights than ANY other caliber, and any company that makes brass, makes 30-06, and a wide range of bullet weights is useful.
Comparing .277" bullets and .308" bullets, the .308" is larger diameter, and if You compared a 150 grain .277" bullet to a 150 grain .308" bullet, at the same velocity, the .308" will transfer more energy due to the diameter, more "knock down" power... BUT, "Knock Down" power (foot pounds of energy) are not the only aspect in a bullets killing power. Bullet construction/performance and shot placement are #1.
The .270 Win. is generally a 30/06 necked down to .270 caliber. Slightly less bullet diameter, in most cases less recoil (in similar rifle and bullet weight), in most cases Flatter Trajectory, and PLENTY killing power.
And, it's a Classic American cartridge.
I would LOVE to see a .270/08. I know someone did it as a wildcat, but I'd like to see that one commercialized, so guys like me can use the same bullets in a smaller cartridge (bigger and more potent than 6.8 SPC) for my son to use.