I would say the one trick that I never leave home with is Two Dollar Window. It is pure eye candy and so easy to perform. Two $1 bills visually change into a single $2 bill. Although there is a dirty little gimmick, it looks clean because you seemingly show empty hands and the two $1 bills on both sides. You can buy 2 dollar window as an instant download or on the Money DVD (with a bunch of other cool tricks).
I used to always have one ready in my wallet, but I guess I must have taken it out or something, because its been missing for a while. Just last night I saw my friend use it, and I realized I need to remake my gimmick.