I sat down after reading the thread and thought "...thats easy my favourite adventure of all time was...????" I then got stuck for a moment. I realised the problem was that there was indeed two sub-types of adventure games. The ones that you played on your own, and the ones that invovled everyone who you knew played adventures.
My favourite solo adventure would have to be Eye of the beholder.
The one kept us going for weeks at school was Guild of thieves for the C64.
Ahh... the memories.