For me it was the 700 5R 308 with Federal GMM 168 grain ammo. It would shoot any where from .25-.5 MOA all day every day. I cant really say that this was 'hunting' ammo but it worked at the time.
The other that stands out in my mind is a 700 VSSF in 308 that I sold. I had never ran factory ammo in it but when the buyer bought it he ran 150 Remington core-lokt ammo in it and put 5 rounds in a hole the size of a dime at 100 yards. Surgical scary.
Sako A7 in 270WSM. Shots the Winchester Supreme 140 gr AccuBond's the best. Best 3 shot group was .375" and best 5 shot group was .75". I've only tried 6 different factory loads from 130-150 grain though.
This rifle shot the Winchester branded ammo a lot better than the Federal Premium ammo though when compared with the same bullets. Maybe the rifle just liked the black Lubalox coating on the Winchester branded bullets better.
I have a mixed bag of factory and custom but I would have to say my best factory rifle with factory ammo would be my 300 WBY Mag Accumark with just about any 180gr ammo I have put thru it. It shoots 180's very well. Funny thing is this is the rifle I talk about selling more than any other I have. Go figure. lol