I’m not sure if ‘Separate Ways/Worlds Apart’ by Journey qualifies as a power ballad, but that’s my nomination.
Amanda by Boston
Surprised it took this many replies to get to Journey. C’mon: “Don’t Stop Believin” (a little fast for a power ballad, but it qualifies). “Lovin, Touchin, Squeezin” is up there with the best power ballads of all time.