The Alexander Arms followed by the Armalite would be my top pic,'s, but I can't afford to shoot .338LM never mind 50BMG.
Saying that, if the rifle and ammo. were free my choice would be the McMillan Tac-50 as it's a very sweet set-up for sure.
I'll give the Noreen an honourable mention for it's simple all inclusive design, plus it's affordable.
Cheers D