FF6 - The ridiculous amount of character customization in FF6 with the Esper system that affects both stats and magic ability is second to none. The core of the gameplay is based around magic and is stupid fun if you lean into it rather than complaining that physical attacks become worthless (unless you know how to stack Sabin, Cyan and Edward). The story also hit me at the perfect time in my life (13 years old in 1994) so there is a strong pull of nostalgia here for me as well.
I think Chrono is technically more polished, but something about the scrappiness of FF6 just speaks to me. Still, both are brilliant and represent the pinnacle of Square Soft development. I don't think they've produced a game I like more than either of these in the last 28 or so years.