Considering I probably have 300+ posts related to the 300 PRC, I might be a little biased, but... 300 PRC.
The only other ones I'd consider:
More Common Rounds:
- 7 SAUM
- 300 Norma
- 300 WSSM
Less Common Rounds:
- 300 Sherman Mag (for a little extra over the 300 PRC)
- 7 or 300 Sherman Short or Sherman Max
- The newer "Mega" line from Sherman looks interesting
As for more common vs. less common rounds, you will find a lot more help from people who have shot the more common (obviously), more commonly available components (obviously), and you won't need to either rent or buy a reamer for your smith - not as obvious.
Of the more common rounds, 7 SAUM would be my #1 choice. 300 Norma is a big round and the components are more expensive (brass, and powder per shot) - and it's a little overkill for a mile. 300 WSSM is interesting and will easily get the job done at a mile, but what do you really get over a 7 SAUM?
Still, with all that said, 300 PRC
