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.
I don't know if there can be a best, but yeah, the Trails games being effectively ffx, but with positioning and even faster is so satisfying.
But then you can go way back to ff1. That battle system is flawed, to say the least. But once you know the ins and outs you can really push it in fun ways.
It's good to mention radient historia too, making the entire battle system be about positioning and moving enemies around on a grid was super fun, it just starts to get really puzzly and fun by the end
But the actual answer if I had to pick goes to Etrian Oddyssy, any of them. When you make a modern game and base it entirely around dungeon crawling combat, the gameplay better be good. And they just nailed it. It's the perfect traditional line based combat it really is. Job systems to keep things fresh, a lot of strategy and the game will kick your butt if you just go in and push attack
Ff1 is a good example. A battle system doesn't need to bo perfect we justneed to have fun in any kind of way. And it's nice they all different.