I chose .45 Colt, and it is my favorite. But other favorites include .45/70, .45 ACP and its high performance iterations, .450 Win Mag (which I've only fired in a revolver) and .454 Casull. I've always thought I would like .458 Win Mag too, but have never owned one.
Not a cartridge, but I'm a fan of .45 muzzleloaders and c&b revolvers, too.