Well - you already have a couple of the best rounds available from the factory already.
At what ranges are looking to try and hit your target?
Without reloading, not many of the factory rounds get up to their potential. I'd vote for 7x30 Waters, 30-30, 45/70, 44 Mag. and 357 MAX - depending on range. (but they all do better when reloaded for best results in the Contender)
Take care,
Bob S.