Am basing my list on what games kept me playing longer than I should (ie, weeks and months in some cases)
1. Elite
Totally enthralling. One of the first games that made you keep playing because the status actually meant something. You had to put in serious hours to get to Elite.
2. Ultima Underworld II
Came on 5 disks and this game was huge (gigantic levels). You kind of got lost in it and it was so atmospheric - even the music was kinda cool.
3. System Shock
The first game that really scared me. Sometime I didn't play at night
4. Doom I & II
What can you say - still remember installing 0.99 and thinking OMG!!
5. Colin McRae Rally II
Having owned a rally car once before this is how I get my fix now.
6. Red Alert
Just played for hours - reaslised I like war games and this was one of the first RTS games that really worked.
Syndicate would be a close second to this.