Former Paris dweller here!
I'm a big fan of the 5e arrondissement. It was the one I lived in in fact. Jardin du Luxembourg, the Pantheon, bars, markets, university areas. I used to go to La Mosquée frequently for dinner or mint tea and cake. It was so beautiful there.
Not far from St Germain and Notre Dame either.
All the galleries obviously. There really aren't any bad ones.
I know people love Montmartre, Sacre Coeur and Pigalle. I like it too, but it isn't my favourite part of Paris.
The Marais is fab too. Totally agree with that recommendation
Kong was all the rage when I lived there. I went a couple of times and it was nice enough, but pricey and occasionally ran into twatty waiting staff and diners (the twatty diners were never Parisian)