To develop on what I said earlier;
PSone Era; MGS1.I put this in the tray expecting nothing, it had been lying there for six whole months after I got ISS 98. I remember always gazing at it saying "Stupid name, stupid game". I was so wrong, instead it all felt so serious despite the Japanese dialogue, and the lack of level-step progress was a breath of fresh air. I spent days on the Heliport, I played it everyday from eating my breakfast (simultaneously) to 4pm.
Worms Armageddon.Two locations, a fixed mode. I still play it. Enough said.
Future Cop LAPD.Dont know what I can add really, it was awesome on Co-op, and awesome in arcade mode. Especially considering that both modes were barely recognisable from eachother. The Transforming in single-player was just awesome.
Toy Story 2The original level set in the house had so many gameplay modes. I used to love the race in the garage around the family car. The sound was eerie but brilliant.
PS2 Era;Ape Escape 2;The game was nice - but its the Monkey Football mini-game that had me hooked. My Brother and I used to always play this, it's the best arcade football game of its generation. Easily.
Tiger Woods 2004;First time to experience the Analogue swing. Everyone I feel should have a TW on their Demo list.
Mashed;Its the perfect party game.
Burnout 3: Takedown;Perhaps my most mind-blowing demo of last gen - I had never experienced the series before this. Now I'm a huge fan. I had never seen crashes like it. The Downtown Northbound (NYC?) track was awesome, and actually had brilliant design unlike Paradise City.
PS3 era;Codemasters!First DiRT, then GRID have really impressed me for their own reasons. They were my first taste at Driving this gen and I was loving it, GRID wasn't much different but the new replay system kept me coming back - Oddly I played the full game of DiRT to the death, yet I was bored of GRID after a couple of seasons.