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.
System Shock 2 is a slow janky ugly mess with inventory tetris and weapon durability, but man I love it. At least, it's slow if you don't go for a goofy sanic psi build lol
countless defensive Breath/Tears fans are now typing...