I remember playing battlefield 1942 long time ago and i have to say that it was and still is one of my favourite games to play even though the graphics are old, it's because of the fun i had and the memories it brings.
Oh, and Dawn of War, used to play with both friends and alone, and it all was fun.
These 2 ones up are my favourite ones, else is good.
Nowadays, i don't really have favourite games, i like all what i buy, or get, most remarkable ones are to say: Garry's Mod since i've played it a lot and met lotta peps, Day of Infamy, Mowas2, RO2, War Thunder...