Well all depends on what you're looking for and if you handload. Let's be honest in 99.9 % of the time one does not think of shooting at elk beyond the 300 yard mark and most can't shoot good enough to ensure a one shoot kill under hunting conditions. The ole 300win mag is a very good round and the least recoil of the three. The weatherby is a good round also I've owned two 300's. The RUM IMHO is the least desireable out of the three. It uses alot more powder than a 300 win mag with what I think is a minimum gain.
If you insist on the latest new offering the 300WSM has propbaly the best performance to powder out of the new offerings.