I'll narrow my claim and say early pioneers of blue-collar USPM. That isn't to say they invented it, but they seemingly formed and wrote music before most of the Californian USPM bands got started. Ice Titan sounds exactly like it could have been from Omen's Battle Cry (pre-dating it significantly and both bands from the same area), and Crionics and Face the Slayer are just about the same. Obviously they share Maiden influence in common, but I think Slayer may have taken it further than Di'Anno Maiden did. Then with Exodus, you have stuff like Warlords and Death Row which is full of USPM riffs and harmonies, and the band was closely linked to Griffin in those days. Agent Steel, Savage Grace, Hexx, etc more or less spawned from the same period but also all came later. Beyond California, Jag Panzer (despite Conklin's voice) played much more traditional non-Maiden-y metal, not even having a second guitarist at the time. Manilla Road was barely even a full metal band in 1983. Helstar played USPM in 1983, their demo arguably one of the purer forms of it for the time, though likely still after Slayer and Exodus (they were abandoning it by early 1983).
You of course still had other more important acts, like Manowar with Battle Hymns on their 1981 demo, unprecedented for its time. That proto-Brocas Helm song from ~1977 was really fucking crazy for its time too.
In a sense, you could say that thrash metal was the logical conclusion of power metal. The bands that stopped with USPM tended to begin their maturation process on the backs of KISS and whatnot, the bands that went beyond it started with Iron Maiden and had a head-start. In another way, you could say that USPM is thrash's younger brother.