What's for you the best turn based battle system?
I'm not a big fan of the ATB system von a lot Final Fantasy game, so for me the best turn based system was a long time the one of Final Fantasy X. It's rather simple having enemies with up to 4 weaknesses and a good system for different alligments for the enemies and resistences. I also like the progression bar that show the action order of friends and enemies. So speed makes a difference. I would still say it's good, but not the best anymore for me.
The best two system for me are not really surprising. One it the one of the Trails of Cold Steel games (i haven't played the newest trails game yet). Adding the area as another factor gives you more possibilities for strategies. So go with more tanky (or fast) characters while the more offensive stays back helps a lot. Also there are enough possiblities to buff defense when a McBurn in the second game just one hits the team. Positioning, buffing, healing, making dmg, they have just done a good job with it. The only downside, it's to hard to break but the 3rd and 4th game have some limitations to present it a bit.
If Trails of Cold Steel is cheating for you because the area doesn't make it a classical turn based game anymore i would say most of the battle systems of Atlus are very good. Regardless of Persona, Shin Megami Tensei or another spinoff, the base of the game with different elements and buff or debuff is often the same. Atlus keeps their games interresting with always different additions to their gameplay. Persona 5 has maybe one of the best for me with baton pass and more addition the futher you get into the game. But i also liked Tokyo Mirrage Session a lot.
Final Fantasy X and Shadow Hearts Covenant have had my favorite battle systems. Especially Shadow Hearts since the judgement ring is a good combination of strategy and skill.