You're literally ******ed.
Abn is the best mage in the world, Arcane/Frost/Fire, it doesn't matter what spec he plays. He's been over 2.8 in all of them.
Jahmilli is "officially" the best mage in the world. He's also the best multi-classer, and no, he doesn't MAIN Spriest, he's playing it right now.
Venruki is another fine mage, he's won multiple tournys around the world, and he's just as good as Jahmilli.
Cartoonz is more of a "for show" player, but I'm willing to be you you'll never come to his MMR.
OP was asking about Frost, dumbass. Why would I tell a Fire Mage to use mastery? If he wanted to play fire, he'd rock HC Cunning, or HC Insignia. The debate is over a FROST trinket, and Soulshifter is easily BiS because of the 3.3k mastery proc. Why OP isn't human, I don't know. If he was, he'd run Soul Casket and HC Soulshifter, and he'd global things in deeps.