I'd suggest you to finish FFX since that game is still unfinished. The PSX versions of FFs 4-6 are very annoying because of the loading times and the overall bad conversion, so I wouldn't recommend playing them. When you've beat FFX, you may want to give FFII DoS a chance. The battle system has been made less tedious than in the Origins (although the action-cancel bug/feature doesn't work anymore), I prefer the translation of DoS to the Origins' translation, there's some new extra features too and the game isn't really easy like FFI DoS. I think FFII DoS is actually quite hard, and the (SPOILER)Souls of Rebirth extra dungeon is extremely hard.