1. Doom 1.
Everyone remembers the first time they played Doom. It just blew me away. Light years ahead of its time - not just the graphics, but the sound (Soundblaster cards were quite rare before Doom, and with one all the alien noises were just terrifying), and gameplay - mind blowing weapons that set the standard (I defy that anyone didn't get a shit eating grin when using the rocket launcher or chainsaw for the first time), hordes of monsters that took a decade to re-experience when full 3d came around, and alternating between gleeful wanton destruction and utterly terrifying (lights go out, lots of noises, trip a wall opening with a multitude of imps spilling out). Birthed an entire genre. Not only that but it popularised network play, level generation, modding (remember the Alien's total conversions?), shareware etc. Absolutely ground breaking in so many ways. Download a modern engine for this and play it again...it really holds up today.
2. Civilisation 2
I dread to think the total time I've put into this series. Steam say 294 hours in Civ 5, and I must have done the same in the others I owned (1,2,4). It's without doubt the most addictive and engrossing game I've ever played. Hard to choose amongst the series but I'll go with 2 as that solved all the niggles of 1 and was just utterly perfect in its day. What I love about Civ is that it's genuinely educational too. The amount of history I've learnt through Civ osmosis is quite impressive.
3. System Shock 2
Sublime; not been bettered by any of the Bioshocks or anything else. Hard as nails; genuinely terrifying, and the best RGP like stats building in a FPS ever - I carried around a rare assault rifle for half the game (using utterly precious inventory space) that I couldn't use until I have a certain weapons ability, and the joy of finally hitting that point and making mincemeat of the bastards that I'd been hiding from until then is one of my greatest gaming memories.
Just sublime. Incredible script (first and only time I've said that about a game); hilarious; mind-alteringly amazing concept, makes you feel incredibly clever. You can't fault it.
5. Syndicate Wars
God this game was good. Rock hard, with unbelievable tension towards the end of an epic level as you couldn't save mid level. I genuinely had dreams where I woke up and thought 'well I'm awake but where are the other 3 of me?'. Again amazing balance of action and building up your characters and weaponry, utterly engrossing. Can't wait for Satellite Reign.
6. Half Live.
Hard to choose but I'll go with 1, mainly because I played network multiplayer in my student house for countless hours of pure joy.
Casual gaming crack that appeals to anyone.
8. Deus Ex
9. Civ 5 - oh go one then, if we're splitting up the series this needs to be on the list.
10. Half Life 2 - ditto