From what I can remember the hardest games I've played (in no particular order) are: I Wanna Be The Guy, God Hand, Monster Hunter, Bayonetta on NSIC Mode (including Secret Boss), IVAN (Beaten using the Slave + Mustard Gas method, otherwise gotten to Oree), Net Hack (didn't really get too into it), Persona 3 (Sleeping Table will forever give me nightmares), the Touhou series.
Of those I'd say God Hand is probably the hardest as I was able to beat it on normal but couldn't touch it on hard mode before I had to return it to the friend I borrowed it from. Compared to God Hand, Demon's Souls is like skipping through a field of daisies.