Uncharted 2 is my favorite demo so far. I don't played that many demos.

Age of Empires 2 Trial. It spawned its own online community with unique gameplay that lasted for well over 5 years.

wow mine is the same as urs. the naruto demo was badass. the game moved way to slow for my liking tho..

Three way tie.

Battlefield: Bad Company 2
God of War III
Heavy Rain.

I've played each multiple times over.

Demo for the original Unreal Tournament.

Soooo addicting.

I don't play demos that often but World of Goo is my pick. I've never played a demo that so completely convinced me that I had to get the full game, if only for the soundtrack.

Screw those free tests of games.  Demoman rules.

Batman AA had a great demo, but I just played the heavy rain demo and that was fantastic. Really took it from a rental to a purchase for me. It does a great job showing the atmosphere and feeling of the game.

Heavy Rain, Dead Rising, and Arkham Asylum all had incredible demos that I played at least 20 times each. Can't decide.

The Metal Gear Solid 2 demo that came with Zone of the Enders. It's shattered to pieces, now. I spent more time playing and re-playing that demo than I did playing most retail games. It was flawless.