8mm Mauser Euro spec or .308.
I would probably have to vote .270 as the ideal North American deer cartridge, but I like guns with a military connection, and .270 is a purely commercial round and only comes in a limited variety of guns (that is, mostly commercial hunting rifles). I'm not someone who only shoots 5 times a year---3 times to sight in and 2 times at deer. I like to shoot.........A LOT.
And there is no arguing the effectiveness of the ol' 30-30 when used within its proper range. Not prestigious enough or macho enough for some I suppose, but it works...
Mauser Werke, Schmidt-Rubin, Remington, Colt, H&K, Weatherby, Browning, Ruger, Marlin, Mossberg, Saiga, S&W, Sig Sauer....a few friends of mine