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 can't beat it on normal in like, hundreds of hours of trying. :sadness:
I only have a few dozen in it because the boss is just anti fun. I’ve heard there’s a really good mod that rebalances everything, might try that eventually.