In my opinion would have to be Valentine's Day. Here is my reasoning for this.
1). Only on Valentine's Day if you are single do feel like a big loser because you don't have anyone to be with.
2). On this day, if you DO have someone to be with, you got to bring them expensive shit, take them expensive places, and do expensive things.
3). For singles, all the good restaurants are packed with couples who are spending "quality time" with each other. This makes US single people have to wait 1-2 hours for a decent steak. WTF?
4). I thought you were supposed to show your love and affection EVERYDAY, in a relationship. Why go all out 1 day, when you do smaller things for 365?
I guess I just don't understand the whole pomp and circumstance of it. When I was with someone, I showed affection and love everyday. So when this time of year came around it was nothing special. I think men around the world should do that instead of hoping that 1 day makes up for an entire years worth of fights, screw ups and excuses.