I vote Reloader25.
I've gotten best performance from my .257wby with RL25...
My 24" Vanguard gets 3,600fps with a 100gr bullet vs. factory 3,450fps, and is equally accurate. Ditto 3,300fps w/120gr bullet vs. 3,150fps from factory.
Likewise, I've gotten better than factory level performance from my .300RUM with RL25. Factory ammo for this is loaded with IMR7828. It's decently accurate at 87.0gr which duplicates both weight and velocity of factory ammo with a 180gr bullet for 3,200fps. But, RL25 is 50-100fps faster and equally accurate at 92.5gr.
But, IMR7828 would be my second choice.
However, if you plan to shoot much, look into some WC-872. It is a surplus milspec powder for the .50BMG and is available from
www.gibrass.com, and
www.widners.com.
There is another similar powder but I can't recall the designation (WC-876?). Either of these are available for ~$9.00lb. That is a lot better than $30.00lb when loading 70-100gr at a time.
Not as good as RL25 or IMR7828, but almost and certainly much cheaper!
My experience is that .50BMG isn't as good in the .300Wby. It's forte is in the .30/378wby. One source tells me that WC-872 and .50BMG are the same powder, just differing lot#'s. That, and US869 is same as WC-8??.
And, there's another new one out that supposed to be the best yet, even better than Retumbo (highest velocities but not as accurate as RL25. It's called PowerPro 4000MR. There's some data for it in the Alliant 2012 phamplet/website.
