As with so many things rock, there's a lot of disagreement as to what the very first one was. Aerosmith's "Dream On" and Styx's "Lady"—both from 1973—have pretty solid claims to the arguably somewhat dubious throne.
But for my money—and with apologies to its predecessor "Hey Jude," with which it shares many traits, not least of which is the same guy singing and playing piano and who may have something to do with the writing—the true original power ballad is this.
Admittedly, due to its one-man-band origins, its a bit less obvious in the original studio recording than in its more famous live version from a few years later.
Now that's a power ballad.
Mullet and sequins and adorbs mugging or no, sweet FSM that man could sing. And, perhaps sometimes overly sweet pop confectionary aside, that was one hell of a band. It's not entirely surprising artists as talented as Joe English and Jimmy McCulloch chafed at being dictated to but, on the other hand, crazy good as they were, they simply were no Paul McCartney. Which, hey, is no crime: semantics aside, there really has only ever been one of 'em.
