I would "re-buy" the S&W 547 9mm revolver I bought new, and then traded for something else, purely for collector value. I've been told only around 10,000 of these guns were produced.
the model I've sold and repurchased the most is the standard frame glock. The couple I have know I'm keeping. As for ones that got away there was a second model match target colt woodsman 90% with box that I could have picked up for less than $500 but you can't have them all and you can't take any of them with you
Well... I may be getting away from the spirit of the OP here but, anyhow, I'd go for G. Washington's pistol pair at the bottom of this linked page... (worth approx. 2 million...)... It's an interesting page...
Lately I have been bitten by a 'collection' like bug whereby I want to collect historical pieces that still have some practicality left in them as well (that is to say not just a pure 'safe queen'). I am now looking to find a 'good deal' on a Browning Hi Power or FEG copy that's reasonable--easier said than done to be sure. It's amazing what even some 'fair' condition historical guns command like, for example, a Luger .