Definitely the slots... Brucey B and Black Pawkeet.
These are mostly an NP sink but some days can turn a profit when maxing out. You have a chance at 3 trophies - Big Losers which is a cumulative score of how much you've spent playing Pawkeet and the Brucey + Pawkeet high score trophies. All three are pretty nice for any collection as well.
Edit: How can I forget the Neopian Lottery... I've won a handful of times. Glorious waking up to 500K+ when hitting 5 numbers.