So far, Cast Performance Bullet Co. has the hardest bullets I have found. I tested the ones I have on hand and they measured 21 BHN on an LBT hardness tester. Beartooth was just a shade behind at 19 BHN. I also tested some of the other bullets advertised as "hard" and they were a disappointing 11-13 depending on brand.
I suggest you get an LBT hardness tester so you can verify some of those claims for yourself. I also found some quantities of Linotype on E-Bay for a very good price and bought 100 lbs. (50 ea. from 2 sources). The BHN on one source measured right at 20, but the other only measured 16. The fellow I got the lower testing batch from had melted down the Linotype and cast it into ingots.
Apparently, it seems that in the process, he skimmed off all of that nasty looking gray scum (antimony), and compromised the hardness of the alloy. Another reason why I think the hardness tester is a good investment.