most accurate factory load for a rem 700p?

As mentioned above Federal Gold Medal and Hornady Match ammo have a really good track record of working well in most Remington 700 platforms.

Just an FYI the question you are really asking what is the most consistent factory loaded ammo. Accuracy is a combination of many factors, good ammo being on of them.

Good luck with the new gun.
 
Gotta go out and shoot some groups. It all depends.
Get 4 different boxes of the high quality match ammo (e.g. Federal and Hornady 168/175gr) in different weights,
and go out and shoot some groups at 100 yds. and 300 yds.
Then see which diet your rifle likes best.
At least try 168gr vs. 175gr, those are readily available, some rifles will have a noticeable difference in groups size between
the two bullet weights.
Have fun.