The first game I ever played is You Don't Know Jack (the original). Bloody amazing for a quiz game, it has creepy atmosphere and it is fucking hilarious. Also, you could literally screw your opponents which forces them to answer the question. It's a great experience, even for people who don't have much knowledge.
Spectre was another really early one I remember, it was kinda like a sequel to Battle Tanks but Spectre has a more interesting concept. Strange atmosphere a long with original enemy's and unique powers. I really need to play this again.
Also, I have to mention Total Annihilation, I played many games before it but this game was and probably still is perfection in the RTS genre.