Comfort is key. Go with a cartridge that you can shoot without the recoil or muzzle blast negatively impacting your marksmanship. Even the .223 Remington, with proper bullet selection, makes for a good whitetail round. In many states, the legal minimum is a 6mm/.243 bullet and that's a fine place to start. Avoid the urge to "upgrade" to a magnum round, unless you're hell bent on mastering it. I shoot most of my deer (and elk, for that matter) with the .280 Remington, though I've taken quite a shine in recent years to the killing ability of the 6.5 Creedmoor when using premium bullets. The .280, which has the .30-06 as its parent case, is a great all-around cartridge (the same goes for the .270 and the '06) and won't beat you up the way the WSMs and other heavy-hitters can. The 6.5 is even milder and easier to shoot and it kills deer just fine.