As fans know, Nami doesn't wield any form of Haki in the story so far. Haki is a power that Rayleigh described to be the manifestation of the user's spirit. Every person in the One Piece world possesses Haki, however, not everyone is able to bring this latent skill out.
There is however a third type of Haki, Conqueror's Haki, that only a few individuals can use. So far, Nami has only mastered one during the timeskip.
According to Eiichiro Oda, Nami is able to hurt Luffy because she's punching his spirit. There was no direct mention of it having to do with Haki. Of course, this was confirmed in the SBS for Volume 43, back when the crew was still in Enies Lobby.
Nico Robin is the archaeologist of the Straw Hat Pirates and one of the crew's strongest fighters. While Robin should've been an Observation or Armament Haki user, given that she spent quite a lot of time around those who wield it during the timeskip, she doesn't equip this skill.
Alongside Jinbe, Sanji, Zoro, and Luffy, Usopp is one of only five Straw Hat Pirates that has unlocked the ability to use Observation Haki.
Roger used advanced forms of conqueror's haki as he imbued his sword with conqueror's haki and clashed with Whitebeard. He also used advanced armament haki for more powerful sword attacks. Additionally, Roger possessed the observation haki, thus, making him the strongest haki user of all time in One Piece.
Zoro is a very powerful fighter and he possesses all three types of Haki. Zoro was able to use Conqueror's Haki unknowingly against Kaido. Biggest Feat: With his newfound power, Zoro managed to defeat the right-hand man of Kaido. However, his biggest feat is scarring Kaido.
Sanji is an extremely skilled Haki user. During the two years he spent at Maiden Island, he trained his usage of both Armament and Observation Haki. Unlike Zoro and Luffy, Sanji wasn't taught this skill by anyone. He managed to learn all by himself and that is certainly an incredible feat.
Monkey D. Luffy has mastered all three types of haki, including Kenbunshoku Haki (Observation Haki), Busoshoku Haki (Armament Haki), and Haoshoku Haki (Conqueror's Haki). He can even use several subforms of the hakis, which makes him one of the most powerful haki users in the One Piece world.
Usopp has awakened his Kenbunshoku Haki during the latter half of the Dressrosa revolt, as he was able to see auras of Luffy, Law, and Sugar, who were in the royal palace, from the old King's Plateau near the Corrida Colosseum.
As fans know, Nami doesn't wield any form of Haki in the story so far. Haki is a power that Rayleigh described to be the manifestation of the user's spirit. Every person in the One Piece world possesses Haki, however, not everyone is able to bring this latent skill out.
He is a specialist of Observation Haki, Ryuo, and also Conqueror's Haki. Among all the Haki types, however, Luffy is especially skilled at using Conqueror's Haki, which is his best Haki type.
However, he survived and transplanted his soul into Nami's sorcery Clima-Tact staff as a vessel. He apologized to Nami and wanted to make it up for his previous betrayal towards her. He later became Nami's official partner.
After the Timeskip
Despite being a skeleton, he grew to 277 cm (9'1"), and remains as the tallest Straw Hat member.
Based on current portrayal I think it's safe to assume Conqueror's Haki is the strongest out of the 3 considering the highest level of achievable skill since it grants you a tool than any character without Conqueror's doesn't have and puts them at a huge disadvantage.
Luffy develops another technique, Fourth Gear (ギア 4 ( フォース ) , Gia Fōsu) (similar to Third Gear), inflating his muscles with air to increase the size of his body (except for his legs) while his limbs are coated in Armament Haki.
Actually not just 1, he can eat 3 devil fruits. Yes.
Heracles is a strange warrior that used to reside in the Greenstone forest in the Boin Archipelago. He trained Usopp in the use of Pop Greens for two years.
The circumstances surrounding Zoro's left eye aren't a complete mystery; at the very least fans know that it happened sometime during the two-year time skip. During this time, each member of the Straw Hat crew committed to training and growing stronger.
Enma is the strongest sword possessed by Zoro of the Straw Hat Pirates right now.