Mine has to be my Vabalathus as Augustus antoninianus, AЄTЄRNITAS AVG reverse. (seller's pics) This was an eBay buy. I was checking the site just before Christmas, and saw it described as an 'Unresearched Roman Coin'. The seller, who has over 15,000 sales and 100% positive feedback, normally sells very common coins (Constantine AE3s, Aurelian antoniniani etc.) in relatively poor condition. I guess this one just slipped the net of the seller, and, as it was Christmas, not many buyers were checking eBay enough to notice it. I was very surprised and pleased to be able to buy it for £31! Looking on coinarchives, most examples in worse condition tend to go for £600 - £800, so I think I scored with this one. It is now the gem of my antoninianus collection What's your best bargain buy been?