Based in your criteria, I think the 6.5 fits the bill best, largely because if the broad range of excellent bullets available. While my primary hunting rifle is a 6.5x284 for medium game to 1000+yards, the 6.5 CM, 260, or 6.5x47 are all excellent rounds for medium range hunting 600-700 yards. As others have noted, I'm also partial to the 6.5x47. It's superbly accurate, easy to reload using the excellent Lapua brass, and delivers better then 20 reloads/case.