6mm, 6.5mm or 7mm are all about equal at this point. All three offer excellent bullet selections, and there are so many different cartridge options in each now that you can pretty much pick a case size to match the velocity range you want to achieve with a given bullet. Biggest thing I look at is who makes brass for a given cartridge. If I can't get Lapua or Norma brass (though I'm here Peterson brass is pretty good as well) I won't consider it for long-range work. The other thing to consider if you're going to be shooting in matches is that a larger caliber bullet is beneficial if they're using best edge scoring.

The .257 caliber is another you could throw in there as some high BC bullets have finally become available for it in recent years, but the selection still pretty slim at this point and some can be quite a bit more expensive. It's a "happy medium" between the 6mm and 6.5mm.