My very favourite video game demo of all time has to be that found with the UK copy of ISS Pro '98 for the Playstation 1, namely Metal Gear Solid. This version only incorporates the first two areas, which you can play through in about five minutes, but we used to squeeze every bit of playability out the disc and stretch it out to half an hour.
There is another, longer MGS demo, but I have yet to get my grubby little grease-coated hands on it. When I do, one can be certain I will perform yet another monologue on its delightfulness. In short, send me all your worthless Metal Gear demos! :P