Honors fall to KESRICK by Lin Carter, hugely subtitled 'AN ADULT FANTASY', because back in the day you had to draw those sorts of distinctions in lines of fire.
Seen from a distance, my initial thought was "is that Robert Plant riding a chicken horse?"
I love how much it looks like a crappy prog rock cover, as if Yes fell on hard times and couldn't afford Roger Dean for their latest opus.
Cover art courtesy of Good Show Sir, a blog dedicated to the, ah, finest examples of fantasy/sf cover art.
I urge you to poke around over there, they have a large store of cautionary tales on display.