Just Cause 2: All the things you can do. It really showed you all the potential of the game. It was trying to do all the possible things on those 30 minutes. When I showed it to a friend, he played it like 4 or 5 times.
Fight Night Round 4: You can a fight a match of Muhamad Ali vs. Mike Tyson! And as a game is wonderful, one of the better boxing simulators I've ever played. This Demo has a little advantage over the full game, and that is blood. It is a shame the full game doesn't have it, because it is really good.
Blur Multiplayer: I liked it because it included many unlocks. The game is really cool. The only downside were connection issues.
And for me, the best demo of all...
Battlefield Bad Company 2: You have no idea how many hours I spent on this demo. How many Battlefield moments. How much destruction. Really. I gathered with my friends to play it. I got new friends! I made a Vodnik fly over me. I killed a stupid UAV pilot that was destroying my team. I blew three helicopters mith a tank (actually I was the passenger, in that tank). I destroyed houses, and walls. I learned to use C4 on tanks... Best two weeks on xbox (before full game)