A Vivid Description
This doesn't mean a five page breakdown of dress and quirks. A few notes is all it needs. "A male Drow with a Lemmy Kilmeister mustache and wearing an amulet" is all the description one recurring foe has ever gotten, but several of my players past and present remember that guy unfondly.
A Legitimate Threat
An opponent has to seem capable and dangerous. This doesn't mean he needs to be able to single-handedly wipe the party, but he has to be dangerous and seem capable of taking someone out. "The Mage-Knight Dorn" was a Teleporting greatsword-wielder, and wreaked havoc on the party and the players from that campaign still hate that guy, even after eighteen years or so.
Who Lives to Fight Another Day
This might be the most important, at least in my experience. The only foes who really strike a chord are the ones who get away. Being able to come back, whether it's in this campaign or a later one, makes for a foe (and a few "Not this asshole again" comments from the peanut gallery.)
Or Takes Their Stuff
Having someone take equipment, money or treasure from the party is like waving the red flag in the face of the proverbial bull. I guarantee my players remember anyone who ever stole from them, ever.
So, for anyone else out there: what makes your group remember their foes?