For me it's probably "Golden Sun" from GBA (and "Golden Sun 2"). I played them at the time on the original GBA at least 3 times each, and on emulators in phones & tablets a few more. I liked the story, the graphics, the combat system, the endless possibilities you could do, the discoverability, hidden corners, the different regions, the global map... If you haven't tried it, I highly recommend it, it's full of nice things.