Although .223 has been effective for many, I've never used it. My minimum threshold is .243 Winchester, although I generally use a .308.
Highest? Don't know. I'd hate to see the DNR restrict that just on size alone, especially for reloaders. Personally, I'd probably not use a centerfire rifle cartridge bigger than the .30-06 on deer, but that's taking energy into consideration; I'd happily use a .35 Remington or a .45-70. But what about pistol cartridges, including the .500 S&W? What about muzzleloaders that go upwards of .70 caliber? I'd hate to see any of these restricted.