Ring of Sustenance: Love! I think that if I could get any magic item to use in my real, actual life, then this is the one! Never have to spend money on food (but you still can if you want). Watching your figure? No problem! Only need to sleep 2 hours per night! Think of all the things I could do with six extra hours per day! Okay, it would probably be six more hours of lazing about, but still!
Holy Avenger! I've been playing since AD&D 1e, 40 years. In all that time I've seen that sword, wanted it, played my fair share of paladins (making sure they avoided the Lawful Stupid stereotype)...and NEVER got a Holy Avenger.
Deck of Many Things: Hate, hate, hate! Others have mentioned its penchant for turning 'normal' PCs into either demigods or....fertiliser. What I hate about it is that no sane person would draw a card that had a 50% chance of killing you! I don't care what the other 50% are like! And draw 4!!! So, a 1 in 16 chance of survival. Unless you are the kind of person who needs to sell your own organs for a fast buck, why would anyone draw? And yet, the sheer unreality, the metagaming of thinking that "it doesn't matter if I draw badly because I can just roll up a new PC, but if I draw well I would have a really powerful PC! It's a shot to nothing!" Well, actual people would not think of themselves as disposable avatars! I hate it!