Honestly, there are too many to choose from, but a few stand out:
1. SB XII - No one has mentioned this game, but for me it was special. My brother was (I think still is, but I'm not quite sure) a HUGE Broncos fan, and he and my dad were rubbing that orange crush crap in my face all week before the game. Words cannot describe the feeling of rubbing it back in their faces afterwards.
2. The 1979 season ender. And Hostile is right, my favorite player of all time was absolutely magical that day. I wish every Cowboys fan could watch that game right now, especially those who haven't seen it. By the way, if I remember correctly, that was the year that the rules changed regarding pass interference. Not coincidentally, Staubach had is greatest statistical year ever. If he played today, there's no doubt in my mind that he would be the best QB in the game.
3. The 1992 NFCCG. I hated the 49ers then. Now, not so much, but then, totally...completely. Everyone talks about how many bandwagon fans the Cowboys have, but I'll tell you something, everyone and their dog was a damn 49er fan at that time. It was sickening. That game was so good in so many ways.