For me it's Chrono Trigger. I always want to play it. I want to show it to my children. I hope it will be regarded as a masterpiece for generations to come.
There are some phenomenal rebalances and new stories for the original PS1 (and PSP if you prefer) engine.
FFT 1.3 is legitimately challenging (there's an easy mode that's worth playing too), ungimps the AI, and makes some fights quite hard. It completely rebalances classes to make them all useful and interesting.