"What one bullet is most accurate in a .300 Win Mag?"
The one which gives the tightest groups.
(Sorry. It's just a character defect.
)
I've always had tight groups from Sierras in my 1:10 '06 sporter. Always sub-MOA. Sometimes down around 1/2 MOA. That's through some 4,000 rounds over a bunch of years.