It probably is not the hardest, but it is a lot of fun and the story is interesting: http://orange.half-life2.com/portal.html
Try "go", I found it to be alot more fun than chess. You can play the online version here: http://www.flyordie.com/board-games/go.html
LOL i dont know. I played god of war (1) on PS2. It was pretty hard. What do you think about daxter on PSP?
the hardest games are thows cheap "unwinnable" games that addictinggames.com publishes all the time cus they cant come up with anything else to publish for the week.