FFX by a lot. The story, setting, music, and turn-based battle just can't be beat for me. (Just X here; X-2 feels too different and I don't care for it.)
FFXII. The setting is awesome, the open world feels amazing to explore, and the gambit system is original and fun.
FFVII Remake. I somehow managed to miss the iconic FFVII, and decided to go into Remake blind—it's fantastic so far, and I'm excited for the next one.
FFXIII and FFXIII-2. Another fantastic setting, and the story is great (although I had to read every datalog entry multiple times to understand it). The paradigm system that shifts battle focus from individual actions to large-scale strategy is unique and interesting. Sadly, FFXIII-3's writing took a nosedive IMO, and FFXIII-2 ends on a cliffhanger, so the series feels unfinished.
FFVIII. It's weird, but it's cute, and I had fun watching the story get crazier and crazier. I don't love the drawing system for magic, so battle isn't my favorite.
Last, I'm only a few hours in, but I can already tell FFXVI is going to be on this list, probably near the top!
Last, I'm only a few hours in, but I can already tell FFXVI is going to be on this list, probably near the top!