I enjoy:
Usopp x Kaya: nice and canon, they're the cutest couple ever. Rich girl x poor boy trope but their problems aren't related to their economy. Odysseus and Penelope situation. She's going to be a doctor to be able to heal him if he ever gets hurt again. I love them your honor.
Usopp x Sanji: lots of food but not canon. I'll do what you can't. Pathetic and distressed bisexuals. They love each other so tenderly and they take care of each other and ughhhhhhh they're wet cats. Also parallelism. I also love them.
BUT I have an amazing, fresh third option, of which I have seen very very few fanarts, and I want the world to acknowledge it.
Usopp x Kaya x Sanji: HEAR ME OUT. Usopp loves Kaya dearly but realizes he's falling for Sanji too. He writes Kaya to let her know bc he won't cheat on her duh. He also sends some photos. Kaya wants that twonk in their relationship right away. Usopp first makes Sanji fall in love with him and then Sanji finds out HE ALSO GETS A SWEET GORGEOUS LADY??? He almost dies that day. He also starts writing letters to Kaya and they click instantly. They all marry and live happily ever after and Usopp gets two blondes and to sleep being held by them.