I would have to say "None". It's been so long since I played a good demo that I don't recall one that made me wanna run out and buy the game. Most of the demos I've played for games I did end up buying I was already very interested in and would of probably bought anyway.
And while this might be slightly off topic I think the increase in no demos and bad demos for games has in part lead to an increase in the amount of piracy. With stores no longer giving full refunds on games that can be returned in the next couple of days (in some cases even unopened!!). Combined with more games being so short you can easily finish them in a couple nights play (Gears of Wars comes to mind as I rented it and co-op with a friend and was surprised when we finished it in a few hours.)
All of this stuff snowballs to where players don't want to risk forking over their 50-70 dollars which is usually non-refundable for a game they have no clue on if they will like. And the demo doesn't do enough to even really show much either. So people pirate games rather then risk wasting their money on something they don't like. And for consoles people also do what me and my friends do which is simply rent the games for a few days. This helps rental places but not so much the developers as sales decrease.
And while this might be slightly off topic I think the increase in no demos and bad demos for games has in part lead to an increase in the amount of piracy. With stores no longer giving full refunds on games that can be returned in the next couple of days (in some cases even unopened!!). Combined with more games being so short you can easily finish them in a couple nights play (Gears of Wars comes to mind as I rented it and co-op with a friend and was surprised when we finished it in a few hours.)
All of this stuff snowballs to where players don't want to risk forking over their 50-70 dollars which is usually non-refundable for a game they have no clue on if they will like. And the demo doesn't do enough to even really show much either. So people pirate games rather then risk wasting their money on something they don't like. And for consoles people also do what me and my friends do which is simply rent the games for a few days. This helps rental places but not so much the developers as sales decrease.