Original Tom Clancy's Splinter Cell. I spent so many hours just to beat that game, it didn't even feel challenging, it was just downright unfair. Most of the time, 1 detection = game over, so you spent a lot of the game *****king up just to learn patrol routes so you could eventually make a perfect run.
Eventually, Chaos Theory got released and I fell in love instantly. Easily one of the best Splinter Cell games to date.