There are several ways that you could look at this:
- We can definitely eliminate purchasable items, loot items, most collection items, and consumable items. There is no limit to how many of these you could get if you had enough time or money.
- Each person can have at most one of each of the tier mastery items (El Rey Roadster, Guerilla Commando, Avispa Machine Gun, Che's Beret, Cocodrilo APC, Cazador Assault Rifle, Barsuk SUV, Ex-KGB Bodyguard, Cossack Armored Vest, ZPR Pulemut, and The Drakon).
- Items that cost Reward Points are probably in pretty short supply.
- The same goes for limited time loot items, though there are probably more of these since you usually get more than one at a time.
If it were me, I'd say it's the items that you get by revaulting the Daily Chance Collection. You can only acquire Daily Chance collection items by either getting 3 out of 5 numbers in the Daily Chance lottery, or by having someone who got them in the lottery to give them to you. The odds of getting 3 out of 5 is about 1 in 582 per ticket you play, and you'd have to do that at least 14 times in order to be able to revault it. The Mafia Wars wiki doesn't even know what that item is called (assuming revaulting the collection is even possible).
EDIT to previous - You cannot revault the lotto collection and why on earth would you ....those things are worth a nice chunk of loot
Ace of Clubs is the rarest loot you can earn doing jobs.