Who is the weakest in Luffy’s team?
Alvida was the first opponent for Luffy to have ever fought and defeated. Her iron mace made her a terror and legend on the East Blue, especially for the strength she commanded. However, Monkey's rubber body made him immune to her blunt attacks.
Luffy's greatest power-up to date is, undoubtedly, Conqueror's Haki coating. During his fight against Kaido, Luffy observed Kaido coating his weapon in Conqueror's Haki. After this realization hit him, he was able to use a crude version of this technique himself and could coat his fists in Conqueror's Haki.
1) Blackbeard With the power of both the Dark-Dark and Tremor-Tremor Fruits, Blackbeard is most likely still above Luffy's level.
Having betrayed Whitebeard and hunted the Straw Hats since Mock Town, Blackbeard is Luffy's greatest remaining threat. Considering the Devil Fruit and powerful crew under his command, the villain won't go down easily regardless of the things Luffy learned and friends he's made.