These would be my top three.
Loose DT Vader. Missing the inner saber, but found him in an ebay lot. Sold the rest of the lot for the cost of the auction, so essentially I have a free DT Vader.
My GDE DS Commander. What a story. Bought it back around 1996/7 for about $160 Canadian (80 GBP), sold it earlier this year due to a financial crisis, then got it back four months later when the buyer graded it for flipping. I busted the ****er out of the AFA case, and it will likely never be sold again.
Finally, my GDE Vader. Autographed in person by Dave Prowse at CII in 2002. No worries - I don't autograph vintage MOC any more, but I have several like this in my collection. This is my most dear prized possession, and would be easily the very last item I would ever even try to sell. Selling this would equate to me leaving the hobby altogether.
Of course, I also have a DT Luke, VC Jawa, Blue Snaggletooth, and plenty more MOC, which could be the highlight of some collector's collections, but these three are my top pieces.
Ian