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
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