The reason Sanji isn't one of the worst is because that would ruin his low profile, sneaky spy role of the crew.
Sanji always keeps low key working behind the scenes in every arc and that pretty much describes him as a character. If the marines were more aware of how he constantly saved the SH Crews ass when they're in a bundle he would be higher but he's not.
Luffy got his bounty from defeating opponents. Namely the warlords.
Zoro got his because he was already renowned as a pirate hunter so he already knew that, then just kept building on that alias.
Sanji however joined the latest of the m3, and with no special title. He wasn't even born in the sea he was recruited, so he had a fresh slate.
That's why Sanji is not a part of the Worst.