1. Luffy obviously.
2. Zoro because apparently Yonko / Pirate King level needs certain overall crew strength.
Now, here is the actual valid reasoning:
3. Sanji
Sanji is both strong and smart, and this trait can make him fulfill many roles.
EVIDENTLY POSSIBLE:
-- Zoro / Swordsman
---- He can use swords due to Germa training.
---- His fight with Wanze is proof.
-- Usopp / Gunner
---- Sanji is shown to wield pistol. Germa childhood training should be enough.
-- Chopper / Doctor
---- With good nutrition and diet, you won't get sick.
---- With recipes like the one he gets from Kamabakka, it's possible that there is healing meal.
-- Robin / Archeologist
---- Sanji is shown to be interested in DF since childhood. It makes sense if he read a lot of knowledge related to everything.
THEORITICALLY POSSIBLE:
-- Vivi / Princess
----- He is a royal, with Germa training he should be knowledgeable enough to be a politician.
-- Yamato / Log Keeper
---- Zeff is shown to have a log book and offered it to Luffy
---- Zeff is Sanji's mentor, him having logkeeping habit makes sense.
-- Brook / Musician
----- Sanji is gentleman type character ala James Bond. Him having able to play instruments makes sense.
-- Franky / Shipwright
----- Sanji can go underwater to fix Sunny. Apparently he's both strong enough and smart enough to understand basic carpentry and engineering.
-- Jinbe / Helmsman
----- Sanji has good enough strength and specialize in CoO, enough to make him understand basic steering.
4. Robin
Robin is smart, independence, and like Sanji, skillful & resourceful.
EVIDENTLY POSSIBLE
-- Nami / Navigator
----- Literally her intro is Robin traverse around early Grand Line by herself and her turle, she obviously can be a navigator.
----- Her job is paper type too, she's smart enough to draw maps.
-- Yamato / Log Keeper
---- Her job is paper type too, she's smart enough to document everything.
THEORITICALLY POSSIBLE
-- Usopp / Gunner
---- She wields gun and is an assassin.
-- Sanji / Cook - Stealth
----- She run around the world by herself since childhood, having basic cooking skill and stealth abilities is normal.
+++ STRENGTH
3. Sanji is evidently already YC2
4. Robin is already Peak Veteran, her DF is actually enough to carry her to YC3 and higher if Oda didn't limit it
+++ BALANCES
With Luffy Zoro alongside Sanji and Robin, SH has:
- 2 Brawl 2 Brain
- 2 DF 2 non-DF
- 2 Frontline 2 Stealth
- 2 Combat 2 Utility
Enough comedy in Sanji Luffy
Enough drama in Sanji Robin
Enough people skills in Sanji Luffy
Enough rivalry in Sanji Zoro
Enough tactical in Sanji Robin
Enough battle strength in Zoro Luffy
Enough pervert and asexual
Enough stern in Zoro Robin, enough sentimental in Luffy Sanji
Strength type swordsman in Zoro, speed type swordsman in Sanji