It depends on what you want it for. For Deer a big Bore out East, like a 45-70, 444, 44 Mag out West a 308, 7-08, 30-06. If you are just going to have fun with it, I suggest a small bore like a 243, 25-06 or a 357 Mag. The obvious choice is a 223, but you already have one.
Good Luck and Good Shooting