I recently started playing a cleric of knowledge and Suggestion has become their "signature spell." It helps that Knowledge Clerics kind of get it as a Channel Divinity at level 6. Fail a wisdom save? You've been Inceptioned.

I love the idea of each character having their own Signature Spell that they cast at every problem. For many wizards, this spell is Fireball, but even then, having a Fireball Guy in a party is its own fun.

I know Wizards get a feature called Signature Spell but no level 20 wizard has ever wanted just one more third level spell slot. I'll make a second post about this probably.

Gust of Wind let's you get up to all sorts of shenanigans.

Easily Vicious Mockery. I have so much fun coming up with quips and it adds so much to the roleplay. Plus I love cracking up everyone else at my table.

I love mirror image! You can grab it pretty early on, so it gives my low level mages something thats visually super cool they can do to set themselves apart a bit

Grease has always been a favorite of mine. All the DMs I have played with have taken the components to mean that it is a flammable oil, and I like to use it to create fire traps everywhere to maneuver enemies into.

I also love Bigby's hand. Interposing Hand in particular has got me out of a few sticky situations. There are quite a few strong contenders for my favorite spell, but I'll have to go with Wall of Force. This one spell has saved my entire party from multiple TPKs, and I consider it the Holy Grail of defensive spells. But while I'm here, I'd like to shout out a few underrated runner-ups. Vortex Warp is a relatively unknown spell, but I've gotten a surprising amount of mileage out of it. Another spell I never see get talked about is an upcasted Bestow Curse. Its selling point is the chance to completely shut down a single opponent, and when it is upcasted, you don't even to use your concentration for it. It's one of the spells that I love having in my back pocket as an emergency.