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 first played Doom in like 2010 and i remember it still blew me away. Yeah running around in circles because you missed a key kinda sucks but everything else about it slaps even today.