The strongest response is usually calm, dismissive, and confident. You want them to look like they're seeking attention while you look completely unfazed.

A few styles:

• Cold: “If that’s what you’re here for, you’ll have to do better than that.”
• Dismissive: “You’re confusing my patience with vulnerability.”
• Cutting: “That’s an embarrassingly transparent attempt to get a reaction.”
• Confident: “You can try. I’m not particularly impressed by threats from strangers.”
• Very dry: “Noted. I’ll try to survive the ordeal.”
• Boundary-setting: “You’re welcome to disagree with me. Harassment is a different matter.”

The one I'd choose for maximum “don't fuck with me” energy without "sounding like a tryhard":

“You’re confusing my patience with vulnerability.”

It makes you sound controlled rather than angry—which is generally much more intimidating than trying to out-insult them.