For me, I'd have to say Old School X-COM (UFO Defense, TFTD, X-COM Apocalypse to an extent). I love them in a way that I just do not enjoy NewCOM, but the interface was quite possibly based on the recovered works of a 13th century scribe, and is very difficult to get used to for an unfamiliar player.
I recently replayed the COD 2 soviet missions. Good stuff, I'd say it's aged quite well since Infinity Ward figured out how COD should feel with this game and basically just made the same game over and over again. Although the enemies were always extremely spongy, I'll give you that