Best in recent memory was my friends and I in our Panty, Stocking, and Garterbelt costumes heading for a private photo shoot at A-kon 23. We had just been informed we had won Best Craftsmanship for the Hall Cosplay Contest and then someone walked past us, stopped, backed up, looked at us and goes "You guys totally win A-kon! Best cosplay of the weekend!"
And a few years ago at A-kon 21, my Kuroshitsuji group couldn't stop getting asked for pictures and being told we were the most accurate group ever. That made us all super happy (and that set of costumes also won a Craftsmanship award).
My last one isn't at a con yet, but I was just recently asked to be a guest judge in the Cosplay Contest at Anime Blues Con in Memphis. This will be the third year for it, and the last two years I have won the divisions I was entered in, so it was really awesome to be asked to judge this year.