I've never had an ace, but there've been several approaches that have been candidates for me, due mostly to my memory: I tend to remember nearly every shot I've ever hit and forget the important stuff (at least in the eyes of non-golfers).
Anyway, one of my favorites would be from the 15th hole at the Heritage course of Marquette Golf Club. It's a picturesque par four with a highly elevated tee and a generous fairway that shrinks as you reach the far end of the landing zone. A small forest of giant, mature maples sits just short left of the green atop a large rocky outcropping - in fact the trees somewhat hang over the left edge of the green.
I pulled my tee shot about 60 yards to the left, leaving myself about 90 yards to the hole if I took a direct route, which would also require me to hit an immediate skyball if I had any chance of clearing the first part of the trees - let alone the last section guarding the green. The pin was in the front left, and from my position I could not see the green or anything even around it. Since it was just a casual round, I put everything into a sand wedge with the ball way up front in my stance. My aunt was watching from the fairway, as were two old-timer regulars in their cart on the 17th (I wasn't far from their green). It cleared the trees, and after listening for some rattling on the way down, I heard none - only the exclamations of my aunt that it had made it to the green, and a moment later that it had gone in.
The old guys couldn't hear her, but later they asked me in a sarcastic tone if I made par on 15. "Nope," I said. "But I did make a 2."
One of my favorite shots I've witnessed from someone else occurred on the 14th of The Heritage, when I was joined by a fellow employee who was still on the clock but bored and asked a bag room employee to cover for him. When we got to the par three, he knocked in a wedge for his only ace, but of course couldn't report it to the pro (our boss) out of fear of losing his job for playing when he was supposed to be working.
My real favorites, however, and what George may or may not be getting at, have been non-hole-outs where I've used creativity to bump and run shots from 50+ yards over mounds and through troughs to get to tough pin locations. Those ones are always a blast and leave me smiling ear to ear.