To be honest, none at all. I've been thinking about this for a couple of days now (well, since I noticed this thread). I sort of hoped someone would mention an item and I'd suddenly remember 'yeah, that's indeed a great item!'. But no, for the life of me I cannot remember a single magic item in D&D that ever meant anything to me.
I remember a game where we discovered a Deck of Many Things. But I don't remember it in a good way.
All in all, in my personal experience as a player magic items in D&D have been either bland or of the kind that were created to screw characters.