We all have heard it.. What's a good response when they say

You wish

Start sobbing and say you do wish...nay, pray...then get down on your knees and beg her to go out with you telling her you'll kill yourself if she doesn't...

I think it depends in the context in which they say it. If they're saying it in a malicious way in which they are the stuck up type , I would just totally ignore it, maybe with a smurkey smile dismissal (If that makes sense), and carry on conversation with the rest of the group. Or you can do a cocky funny ''Oh yeah totally, every birthday I wish specifically for you over all the girls in the world, I wish for you sweetheart''

I'm not sure to be honest, normally when my female friends say this to me or I may even say it to them as a joke, if we're having some sort of banter. I normally do the cocky thing, or I say something along the lines of wishing for a celebrity and that could be both sides of the spectrum whether I say Susan Boyle or Jessica Alba etc.

I hope this gives you a couple of ideas.

Can't really think of a snappy comeback. The best thing I can come up with is something like 'not really' . Or something else that indicates that you don't want her as badly as she's implying.

Her: You wish
Me: Don't flatter yourself

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Keep people off-balance and in the dark by never revealing the purpose behind your actions. - Robert Greene

Yes!
That's a good one