If your party is in melee with multiple enemies and you want to do the typical god wizard thing, Slow is your best option. Sure, catching them with Hypnotic Pattern is way better but there are situations where you just can't to do that (low ini, being surprised etc).
Slow is only a candidate for 'best spell in your arsenal' when:
A) Your frontline is completely scrambled, making most AoE spells difficult to use even if the frontline spends a turn reorganizing.
B) You're fighting monsters that simultaneously have a middling WIS save and can be meaningfully screwed-over by restricting their spells/multiattack.
C) The monsters are in that Goldilocks zone of not being pasted by raw damage in one or two rounds while still not being over-CR'd to be a horde.
Do those situations occur? Yes, they do. My party got ambushed by a pack of Cloakers the other day. But for the most part? You're better off with an upcasted Blindness/Deafness, Hypnotic Pattern, or Fear. I only have so many spells prepared.
Thanks for the input. But to be honest, I'm not convinced. Why spending actions to get creatures back into the sphere when you can cast a spell that does not allow for consecutive saves in the first place (like Hypnotic Pattern)? The forced movement aspect does not really convince me. 30 ft just isn't enough, most monsters can cross that distance and attack in one round.
Well, for one, there aren't many spells that don't allow consecutive saves at part of the game. There's pretty Levitate and Banishment. Hypnotic Pattern doesn't really count, since A) chaff mobs can wake up the other mobs as an action and B) if you want to do damage, you have to break the effect. Don't get me wrong, it's a very good spell, but it's not difficult to find spells even of that level that oftentimes does the job better. Such as Fear.
But as for Watery Sphere:
A) The restraining has an end-of-turn save. So the enemy won't be making the save and just running up and wailing on you.
B) With a minimal amount of common sense, it completely locks out spellcasting. I suppose you could make the argument that you can do verbal components underwater, but that's against RAW and RAI.
C) It's repeatable and reusable. If you fail or an enemy saves, you get to try again. Once your sharpshooting Eldritch Knight completely obliterates one of your held targets, you can move onto the next one. It's also easy on the spell slots.
D) It's 30 feet of multiple-target forced movement. That kind of forced movement (both the distance or number of targets) is surprisingly rare in 5E D&D. Which is understandable, since 5E D&D also publishes a lot of killer zone effects balanced around the foe getting a chance to bodily escape.
Watery Sphere's only real failing is its size limitations and targeting STR saves. It's not the perfect 4th-level spell, but it holds its own against heavy-hitters like Resilient Sphere and Polymorph and Black Tentacles and Banishment.
Well, you still need to hit. Against AC 19 assuming a reasonable level, the spell has a 40% chance to do nothing.
In my defense, dragons have huge amounts of AC for their CR. Aside from the Iron Golem, you generally only see 19 AC on CR 20+ creatures. Considering the only alternative is to try to punch through their legendary resistance or target a check, I'll take it.
