I was recently listening to a particular album that's been gathering dust in my collection for literally years, and suddenly I felt a wave of nostalgia connecting this to the PC demo for the original Grand Theft Auto - because that didn't have a soundtrack, I'd stick a particularly long CD in while going on rampages, meaning the two are now apparently forever linked. On a slight tangent, a more contemporary example of this happening is my associating Watch_Dogs with pretty much the entire back catalogue of ANb (that's Agoraphoric Nosebleed for those of you who don't recognise their shortened name) due to me not really liking anything licensed for that game apart from perhaps one or two songs... I know, it's a strange mix (and probably the reason I'm so aggressive when encountering other players online)!
My recent most played demo was for Forza 6. I beat it 3 times, it's actually a very well made demo. I would buy the game now, but I don't have the necessary funds, will have to wait until the game becomes cheaper.
I remember spending hours with Wolfenstien 3D and Out of the World (named as Another World outside USA) demos because back then (mid 90s) the only computer I had that were supported were Macintosh. C64 were all but extinct in stores and Macintosh software were hard to get outside mail order and local BBS.
The wait till the 15th has been taking ages, usually days just go by, but this.. this is taking the cake. I bought the whole bundle with the console
Another demo disc I came across that I played a ton of was the Spiderman PC demo that came with some cereal. It was like the same game that hit the PS1 and DC, but the sleeve looked all like the movie. You make it up to a fight with Rhino, then repeat.