What is the most challenging but fun RPG for the PS2? What is the most unique/unusual?

I’m looking for a different RPG game to play. It can be old or new. I’m also curious what your opinions are about challenging and unique games. Thanks a lot.
Also, if you can think of any challenging or unique Gamecube RPGs, feel free to list those as well.
I should add I’ve played every FF for the PS2, as well as DQ8, both Dark Cloud games, Wild Arms 3, and Star Ocean – TtEoT