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.
There's something hypnotic about the Morrowind cadence of moving forward and backing up repeatedly until a spear hit lands... it's so satisfying.
I would have a gambling addiction if I gambled, wouldn't I?