panhandlepr, in standard calibers, I'd opt for a 6mm Remington. I have pals that shoot them at paper out to 1K with very good results. Nice cartridge. There's nothing wrong with a good .243 Remmy either.
For a wildcat, if you don't balk at a barrel burner, a 6-284. Great brass, lots of extras, fierce performance.
There are many good bullets for both. I'd consider a custom chamber to get the most out of heavier bullets, quality bedding, & a nice crown.