Out of that list I would have to say Ys Celceta.
In regards to difficulty though, the fact that you have a 3 member party, and the ability to spam potions removes any real challenge from the game. I know Falcom had to make the game and trophies accessible to casuals, so Ys veterans like myself just had to self-impose restrictions.
Your challenge, should you choose to accept it: Nightmare run with a limit of 1 potion per boss fight. Old school style. Other challenge: After beating the game, start a nightmare game, and when you first take control of Adol at the inn, do the boss run right there. Not only is Adol solo, but you will have pre-set gear for every fight, and a limited number of potions. Adol solo = true Ys. And yes, it's possible to get through any fight with 0 damage by using flash move and flash guards. I recommend flash guarding for crits to reduce fight times, similar to Ys Seven.
