I think it's surprising that the cast that deals the most damage is actually
Dionysius. At first glance yeah, it's kinda clunky to use since it is slow, but most enemies/boss aren't exactly fast either. But then you get Demeter's duo boon Ice Wine and holy shit his cast became very fast, hit a wide area and inflict chill too. Mix with some Zeus, hangover area also strike lightning. At this point, if not for waiting for your cast to return, there's no point in attacking normally. 251x6 flurry cast, I'm playing range attack rather than close combat.
That's at level 4, no other cast even reach that level of damage at higher level, and don't forget Chill because I have Ice Wine.
Artemis' cast is kinda niche, good for single enemies, terrible for crowds.
Demeter's cast looked nice at a glance, but with moving boss, the dps is not optimal.
Zeus' cast seemed 2nd best. It does a decent dps, and it bounces/homes across multiple foes. The speed is decent too.
Ares' cast need some skill to use. Use it when enemies are stationary or charging, use it up close, otherwise it's wasted.