I'll toss my vote onto the pile.

ANYTHING from 6.5mm up to .338 (this includes nearly all the popular cartridges...the 243 and .44mag rifle being the only 'good' deer cartridges I think it leaves out)

The type of bullet you shoot is going to matter a whole lot more than its diameter...or even its weight.

However, If I was going to have only one cartridge to shoot only deer with for the rest of my life I would choose a modern 6.5X55swede-OR-7X57mauser or a .260-OR-.280 (flip a coin, there the same difference) in a carbine rotary lever action...which I'm pretty sure haven't been made since the Savage M99's.