Wouldn't it be the one that's killed the most deer? The 30-30 of course.
Man there are sooooooooo many good choices out there for deer. I currently use a .30-06 beause it's my all around rifle and works well. I like the 30-06 and it's varients a lot like the .25-06, .270, .280. I also think the .308 and it's brother the 7mm-08 rate just as well.