Hypnotic Pattern.

FIreball is good, don't get me wrong. AoE damage with absolutely fucked range and radius is hard to argue with.

But Hypnotic Pattern coming off a Bard (due to Cutting/Unsettling Words, and possibly the horrifyingly overpowered Instrument of the Bards) completely destroys any encounter it touches.

It effectively does one of three things:

-At worst, it burns a Legendary Resistance

-Second worst, it wastes several enemy actions as they all need to spend turns waking each other up instead of doing damage

-Likely: Turns any swarm encounter into a bunch of very easy 'party against one mook' fights

-Likely: Allows you to just skip a boss' turn