Something in 6mm or 6.5mm. So basically .243 or 6.5 Creedmoor. The .260 is heading for dead as is the 6.5x55, but if you want to there's nothing stopping you - they're just fashion statements that say "I like old things that aren't designed that well". There will be no practical difference except for extra rifle weight on the 6.5x55.
If you're going for larger animals (elk, large bears, etc.), it would definitely be a 6.5mm as they're the only option that offers reasonably high sectional density (and thus potential for penetration) at the bullet weight you're wanting to shoot.
