I just like to be as aggressively polite and friendly as possible out of spite. If the person is just having a bad day, being kind in response generally calms them down and deescalates the whole thing.

If they’re trying to pick a fight, they get more and more pissed and it’s hilarious to watch because I just smile and agree and they blow UP.

I wouldn’t say necessarily I have a “go to” to respond, but I defend myself in a polite manner. Or when there’s things I’m getting yelled at that are out of my jurisdiction. For example, the other day some lady was sitting on the high top counter and apparently there were ants running along the windowsill. She flipped out and said this isn’t acceptable etc etc. My response was “I’ll let my supervisor know🙂”

Get comfortable with your expertise. Generally you know more about your job than customers. (If you don't know, just reassert "Let me check with a manager on that for you," until they stop demanding your attention so you can go get your manager.)

Reassert things you know to be true in a polite way. Be apologetic. Kill them with kindness.

Honestly there really is no reason for comebacks, if someone is hurting your feelings let your partners know. Your co workers are called partners for a reason, and they're all there to support you. Obviously in a day to day setting you should be standing up for yourself, but I wouldnt recommend talking back to customers in a retail setting, especially I'd the company is all about "making the moment right," and "assuming the best."

TLDR, if you're mean you'll get fired, just talk to your partners if a customer is bothering you.

You should either get a shift or another partner to finish the interaction with them as it is a place of business and ‘come backs’ are not beneficial in any way. I understand frustration with customers, however; meeting a less than ideal customer’s attitude with attitude is a quick way to escalate a situation. You can be super nice, say “thank you so much I hope you have an absolutely wonderful day!” In an annoyingly positive way, because they won’t want to hear it and it’ll irritate them but it’ll still be within the parameters of your job, just be careful with the tone of how you say it or they can claim you were being blatantly sarcastic and rude. Many DT customers can be rude at the speaker box and repeat themselves over and over when you have everything right, to which you could say “yeah I have that.” Again, careful with the tone. No offense OP but this is a job, not a social setting, and you are paid to interact with customers in only the best ways.