Mine, is Browbeat. I love cards which force people into nasty decisions. I'm surprised more burn players don't include this one in their deck. I love dropping this on turn 3 or 4.
Kill you now or kill you later.
http://gatherer.wizards.com/Pages/Card/Details.aspx?multiverseid=36038
Great card, great artwork, great idea.
Opponent choose, I win.