It depends on what the most difficult means. Pre-GBA FF1 was hard in a way that RNG can mess you up anywhere. Ice Cave was especially unforgiving, as you can get ambushed by 9 sorcerers/mages at any time, and they'll kill your party before you get a single turn. This is less of an issue in post-GBA versions since you can save anywhere. FFs II and III aren't that hard, but they're very tedious in the way how much they require grinding.

IV is the hardest one for me. I haven't played the DS version, but the Japanese Hard Type version for SNES was really hard for both random battles and boss fights. The battle speed seems to affect the game a lot, as my levels had to be in the high 60's to beat the last boss on the fastest battle speed, while I can do it 10 levels lower on the default speed.

V is quite tough at times, but it's beatable with minimal grinding if you can come up with good ability setups for your party. I thought VI, VII and VIII were the easiest games in the series. IX isn't too hard, but (SPOILER)Necron and Ozma are hard in their own ways since those battles are heavily luck-based. I don't think X and X-2 were that hard, but some of X's boss battles are very frustrating and take ages to retry because of unskippable cutscenes before the fights (I'm looking at you, (SPOILER)Seymour Flux, Yunalesca and Braska's Final Aeon!). XII started tough for me since the characters gain levels very slowly, but after the beginning parts, I didn't think the main game was that hard.

FFXIII had some pretty tough battles, and at some point, every battle felt like a mini-boss, but at least you can retry every battle, so Game Over isn't really an issue. XIII-2 was a lot easier, but some of the DLC battles were amazingly tough. I've only beat LR on the Easy difficulty, and it wasn't too hard. Tactics has a very steep learning curve, but it didn't feel that hard when you've got the hang of the battle system.