I’d always say Paris but it really depends on what you like to do
The haters will say it’s busy, overpriced, dirty (which always puzzles me, it always looks cleaner than most UK cities)
I’ll tell you it’s impressive, beautiful, stylish, relaxing (if you don’t drive). There’s far too much to see in a day so choose a few things that you’d like to do.
Yes you could end up being overcharged for almost anything, but do a small amount of research and you can easily find good food that’s reasonably priced. Don’t expect the locals to bend over backwards to be nice 😀