Puzzle Quest took all my attention from everything else I was playing. It's insanely addictive. The Xbox Live Arcade version is also the cheapest for its release date. (~$15)
Though I like the interface on the PC-demo version better, since a mouse or stylus is ideal for it, but I just can't stop playing it.
It was out on the DS and PSP for a while, but the 360 and came out recently.
Best aspects of both RPG and Puzzle game elements, hands down. Even the story doesn't feel tacked on.