I still play the first Unreal...
There is still a fairly active singleplayer mod community for this and UT. I was just checking out the progress with this. 
Well, OldUnreal, UnrealTextures and UnrealSP are on my favorite bar, soooo...

It's just fun that the 3 first games I ever bought (in order : Unreal, FreeSpace and Homeworld) are among those who have, even years later, great communities.
For Half-Life 1 : I found it inventive, not just the kill ! kill ! kill ! game, very pleasant to play (finished it at least 5 times), but a little bit too...clean (except in Xen, of course) compared to Unreal and his sinister atmosphere. I like to be scared and I'm pretty sure that the Skaarjs took me at least one year of my life

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For Half-Life 2 : I really prefer what have been done with his engine than the game itself (PORTAL !!! )
BUT ! This is only my opinion, so don't kill me.
To strictly stay within the subject, I think that the games we played some years ago are the same than the games our children play today : great memories for the years to come, nostalgia... That's for THIS that GoG.com was created, after all. It don't prevent me for liking actual games (I pretty enjoyed Batman : Arkham Asylum, for example), but it's true that our first gaming experiences often remains in our mind.