Girardhelli brownie mix. Not claiming it's the best ever but it's 100x better than the Betty Crocker or Duncan Hines crap. But there's also the barrier of 5-10 mins prep and 25+ mins baking so it's not instant gratification y'know?
Once you bake a pan though it's pretty great for the next day or two. Assuming you're not too concerned with your waistline.
Edit: Oh and a box is like $3 at Big Lots. But you do have to supply an egg and cooking oil.