6.5-284. Runs well in a long action. Factory ammo in a pinch. I am shooting 140's over 3k. Very accurate. Great bullet selection. Barrel life is actually quite good with slower powders (H1000, Retumbo). Lapua brass.
Get an 8 twist long barrel, throated for longer bullets to take advantage of long action. Can do one of the others listed above, but a little more work involved.
That being said, a 6.5 PRC with a long throat for all those new, longer bullets would be great as well.