Most of the games I own are on DS, and all my favorites are on that system:
1. BIONICLE Heroes. This was my first game I ever played and it's still my favorite.
2. LEGO Harry Potter Years 1-4 and 5-7. These games are just works of art. They're a lot different from other LEGO games on DS, but still similar in many aspects. Very refreshing.
3. All other LEGO games. There's just something about them that I like. Maybe it's the fact that I love playing with LEGO bricks.
4. TransFormers: War for Cybertron Autobots and Decepticons. I have Revenge of the Fallen Autobots on the DS as well, but these games are far above that one!
5. TransFormers: Revenge of the Fallen Autobots. That doesn't keep it off the list, however!

I don't have any Harry Potter games other than the LEGO ones, but I plan to get Half-Blood Prince from GameStop tomorrow...