Lessee...at first, I thought money. Because money is good. Not to mention with money I can buy most other things.
But then after I thought a bit, I came to the conclusion that my reward of work is that...it's done! I mean, if it's some big hulking nasty bit of work that you dread doing, when you're done, you don't have to worry about it any more. This could branch off of the good deed selection, but I think it's more about my own selfish wants.
Oh, and when it's done...nobody will yell at me about doing it.