Well, there are a LOT more copies of the cards that went on to be reprinted in Revised than any of the other cards in A/B/U.
So a card, even if it sucks, is more likely to be used if it has been reprinted. I've seen a lot of inexperienced players build decks with crap like the Laces. And then some of the reprinted cards have seen more use in drafts. We also have the cards that were not reprinted, but are actually good, like Illusionary Mask or Word of Command. I think it's safe to say that most cards in A/B/U are going to have seen a lot more play than the few that were never reprinted AND suck.
There are also three ante cards in this set. I don't know how to handle these, because ante seems to have lost nearly all of its popularity before I even started playing. I've never played for ante, but I have used the cards with my Relentless Pony deck and I used to play with people that had played ante (and used the ante cards). Also, you get those jokers that like to put twenty ante cards into a deck and remove them before starting the game just like the cards instruct you to do. I think the least used cards in the game just might be the ante cards. If that's the case, I'd guess Darkpact is the least used of the original three. Contract from Below is basically the best card ever printed and Demonic Attorney seems like a fun way to frustrate an opponent (do you concede because I can win soon and want a free card, or do you think I'm bluffing?), so Darkpact seems the least likely to be used.
If we're ignoring ante cards, then I'd narrow it down to those cards that aren't particularly good, were never reprinted past Unlimited, and are rares (because there are fewer copies of them available and that was the case back in the day as well). I might have missed a couple, but I'd include the following in that list...
Natural Selection
Lich
False Orders
Blaze of Glory
Cyclopean Tomb
Chaos Orb
None of these cards has seen too much use, but which one has seen the least use? Well, Chaos Orb is something fun and unique, so I'd cross it off the list. All of the others are lackluster, but Blaze of Glory and False Orders provide generally useful combat effects, so I'm sure they've seen a little use. Natural Selection isn't good, but it's not terrible. Cyclopean Tomb is pretty bad, but not compared to Lich.
Lich is an expensive card that hurts you instead of helping you. It's possible to use it to your advantage, like with the TurboLich combo (Lich, Glacial Chasm, Fastbond, and Zuran Orb), but such decks are few and far between.