For single mobs basically
Huton > suiton > raiton
3+
Huton > Doton (unless mobs are almost dead) > Katon
2 mobs can be tricky since it depends how much hp they have.
2 Mobs isn't tricky at all. If you know they won't be moved, and also that they won't die for 21 seconds(requisite tic duration to beat Katon), you use Doton. If neither of those is the case, use Katon. Since they buffed Katon, it's 250 potency.
I overuse Katon, because I am lazy, but also because most tanks are ***and love to move + monsters don't always clump right. It's actually uncommon to get a perfect doton down; Bards can relate with Flaming Arrow.
Basically though, you want to enter pulls doing Katon > Kassatsu > Doton.
Also though, one thing that really grinds my gears is that they removed Katon from the auto crit list for Kassatsu, granted it'll never be as good as Ideal Doton placement, but still makes me mad. Real fun to see 10+ targets all get crit. I guess they figured they'd leave the crit thing for Doton, because it's so easy for Doton to get screwed over.