[YankeeFan]

Dave Righetti's no-hitter vs Red Sox at Yankee Stadium on George Steinbrenner's birthday, July 4, 1983.
I still have my ticket stubs, parking pass and Kodak photo's taken during and immediately after the game.

2nd place (maybe tied for first): September 30, 1973, the last game at the original House That Ruth Built. I still have the ticket stubs and program to that one.
I remember using by Boy Scout pocket knife to slowly cut through a wooden slat in my seat. Who knew they'd be selling entire seats at Korvette's for 19.99!
I also remember yelling "Houk must go!" with the crowd towards the end of the game, and then feeling bad the next day when I heard that he was fired.