Hmm...That is a tough one, Dark souls 2 is definitely up there for me, Quite a few of the bosses gave me a rough time especially the first real boss with me being a magic fan .I suppose Baten Kaitos Eternal Wings and the Lost Ocean. Completing all the quests and making a good stable deck for each party member really takes alot out of ya. and certain bosses had me in a tight spot.
More recent one is faith all 3 games, had a blast figuring out the boss patterns and finding all the notes and unholy trilogy adds a good ammount to the first 2 games.
the mega man zero games were an interesting set 1-3 and 4 had a really good story that despite the many reloads had me at the edge of my seat,
system shock was so very much fun as was 2, sadly my laptop is way too weak to try the remake of 1 but the remaster despite being dated is wonderful fun. I really dont get why people hated the og cyber areas. its not as bad as they claimed about gettin lost as for the rest of my catalogue of old games not sure theyre what people would call hard, persay.