I'm going to have to give it more thought before I answer... The freshest one in my mind is Paul Casey's hole in one to finish off a Ryder Cup Match. But I'll have to get my memory going.
For me the circumstances have as much to do with the greatness factor then the shot itself. If I guy holed a 240 approach on a par 5 in the middle of a round of 78 and he finished no where near the lead, it doesn't make it on my greatest scale.