You could say something that's complimentary about the other girl, without actually implying that you think she's prettier or whatever.
Girlfriend: "Do you think she looks better than me?"
You: "Her boots are neat, and those highlights look kinda cool, but nothing that could ever surpass you in any way."
That way, you're saying something true and nice about the other girl, but also reminding your ladyfriend that she's your number 1 no matter what. She might also be impressed at you talking about fashion stuff.