It depends on the rifle for me. Some factory ammo will group extremely well in one and not another. But with some trial and error you can find the one your rifle shoots best. Federal blue box, Federal Fusion, Hornady American Whitetail have worked well for me. I have 270 that really likes 130gr Fiocchi Field Dynamics. It shoots it better than anything else.
When I get a new rifle I test at least 5 or 6 different types of ammo in it. Its easy to tell the winner, but for each particular rifle it's usually a different brand of ammo. It might take some time but the results are worth it.
