If I had to, then its gonna be Nami and Sanji if I'm being honest. Ussop being lucky comes in at a close third, so he can stay.
My reasons for kicking them out doesn't have anything to do with their roles nor their strength but how much i enjoy their writing and character in the story, especially post time skip.
Nami:
Outside of her role as the navigator, Nami for the longest time has always been one of the weakest straw hats while at the same time treats more poorly of her fellow crewmates for no good reason, especially Luffy, Zoro and Sanji. This 'gag' is very rarely funny to me and its so bad that only Zoro is one who's constantly annoyed and pushes back against Nami's crap. Luffy is too stupid most of the time to care, while Sanji is Sanji. A lot of times Nami just wants to be the one making the decisions, even when at they are at times not the most sensible and fair, i.e how to handle Ussop's return in the Water Seven Arc. This inverse relationship of Nami being one of the weakest crew members, while at the same time having some of the most authority and say in the crew a lot of times never sat right with me. Because i find a lot of times Nami's bases her judgment on her emotions, and when she makes a wrong call and realizes she can't help the situation all she can do is just weep and cry, leaving Luffy and the others to come in and fix the problem.
Nami has Sanji under her thumb all the time, yet the one time where she could have put this to good use, AKA in WCI to get Sanji sorted out, she failed. She was hopeless in that moment and just cried, was unable to do anything in trying to stop Sanji from kicking Luffy in the face.
A character like Nami who has so much power and authority on the crew, while can also be quite careless and irresponsible in using that power and authority, i feel like would be causing unnecessary frictions and problems in the crew, however small they may be.
Sanji:
I'm finding i'm enjoying Sanji a lot less after the time skip. Oda has gone way overboard with the annoying gags to the point where in post time skip, the moments i enjoy Sanji are the moments when he's not having a nosebleed, being a perv or overreacting all the time. Oda could honestly tone it down a notch and it would have been much better for me. He does it so much to Sanji now it feels like to me that Oda has turned Sanji into a caricature and i don't like it. Nosebleed every time he meets a hot girl, constantly baiting and picking fights with Zoro over stupid crap, while also being a bit rude, immature and insensitive a lot of the times. Sometimes Sanji really just comes across as being absolutely insufferable. I didn't like it when he told Jinbe to die after he told his story, however much Sanji really meant those words. I didn't like it when Sanji kicked a already beaten and tired Luffy in the face in WCI, after what Luffy went through just to be able to meet him and talk about his problems. I didn't like it when Oda made Sanji a absolute fool in WCI as he was duped and tricked by everyone, from the Vinsmokes, to Pudding, to Big Mom- when Sanji is usually the more smarter and objective of the straw hats (remember how Sanji made the navy ships crash into each other when they were escaping enies lobby? < that Sanji was absent in the WCI Arc.)
The two year time-skip has also somehow made Sanji more immature than before to the point that before he was gleefully celebrating his 10 million higher bounty over Zoro in after meeting him again in Wano, to also being jealous of Jinbe during the raid on Onigashima. Sanji's jealousy, overreactions, pettiness, rudeness and insecurity makes him look and come across as a 12 year old in a adult's body. While his personality may be fun to read in a book, i can't help but think that someone with sanji's certain special mix of bad qualities would be absolutely insufferable in real life.
If i were Luffy, i don't think that i would be able to stand a character like Sanji long enough to reach the One Piece.
So yeah if i were Luffy and i had to kick out 2 people, then it would be Nami and Sanji
I don't really care about strength, usefulness, or popularity in this discussion, as i think that whether i can stand or get along with them is more important.