I own a Tikka T3 Varmint Stainless that is the most accurate rifle I have.....but NOT "Out-of-the-box". It took 125-150 rounds for it to settle down. I was impressed with it initially because of the acceptable accuracy and the slick action. But it was definitely not the most accurate rifle I have seen "out-of-the-box". Now, it is very, very impressive. It has had no work done except for the trigger being adjusted.
I own a bunch of Rugers, and 4 of them are VT Target models that are scary accurate....and they were that way initially. Now, not all of my Rugers started that way, but these 4 did. They both have several hundred rounds through them and they have never failed to impress me. They are box-stock weapons with no changes or adjustments. They are a .223, 2-.204s, and a .25/06.
If I had to make a choice, it would probably be Tikka. But that being said, I have never owned one of the more expensive rifles like a Cooper or similar. And I don't consider any of my ARs as being "stock" guns, so they aren't being considered.