There is no "best" SA cartridge, but there are plenty of good ones. I shoot the 260, but would be quite happy with many of the others. Just get the 308. It'll do all you need, and ammo is in every Bubba bait and feed store in America.
If that's too boring for you, then I'll suggest the 6.5-284, which is basically equal to the 270. But you had better be a reloader.
