Robin - did i need to say?? Literally the key to Laughtale, Roger would never be PK without Oden help.
Sanji - Most Versatility SH, the cook of the crew, strategist, one of the main fighters etc, always comes in clutch etc.
Zoro - The powerhouse.
So far Nami role as navigator is lackluster and barely visible, the crew always reach the destined island without her lead specifically.........
She's just been used as fanservice, comedy, beating Luffy/Sanji.
Towards the end, her usefulness and worth may pay off i hope.