1. He is very useful for Oda (+25)
In meta sense, Oda uses Sanji in various situations and pairings. Oda can fit the fighters. Oda can fit him as the gag character. Oda can make him do brain stuff AND brawl stuff. Oda can use him as stepping stone AND hype character himself. Oda can fit him with the girls. Oda can fit him with the drama.
You get what I mean, he is useful to have as a character in your story.
2. He is very useful for the OP plot & character (+25)
Sanji is one of the few characters who progress the story. Him saving people left and right, do spy stuff, benefits the SH a lot.
Sanji is also one of the few characters, if not the only one, who is very useful IN combat and OUT of combat.
Everyday, he cooks.
Outside everyday life, he gives out nice advice, he does the comedic bits, and sometimes join the drama.
Before, mid, or after battle, he does spy stuff & sometimes saves the day.
In battle, he always fought top 5 characters.
3. He has a complete character & characteristics (+50)
Nuff said. Watch Ohara's video about Sanji and hero phase for details.
---
A. He limited himself
He can fight using his hands, for sure this will increase his combat strength.
He can fight with blade, for sure this will increase his combat strength.
He can use guns too.
He refuses to at least defend himself against woman.
All in all, it's nice if you have principles, but this way, you're weaker and thus helps the SH less.
B. He doesn't work hard
Imagine if he at least work hard half the time of Zoro's.
---
All in all, point 1 2 3 makes him a 100/100.
Point A B is around 20, so in total, Sanji is around 80/100 for me.