In One Piece, the World Government is a powerful entity that controls most of the world and finds itself in perpetual battle with pirates. While their primary military force is the Marines, they also have 10 known Cipher Pol units which have different specialties. The secretive CP9's members are all assassins, and are the major antagonists of the Water 7 and Enies Lobby arcs. While they are led at the time by Spandam, the strongest among their ranks is the cold and callous Rob Lucci.
After his defeat by Luffy at Enies Lobby, Lucci left CP9 along with Spandam and another member, Kaku, and joined CP0, the World Government's strongest Cipher Pol unit. This signals his strength continuing to be held in high esteem by his superiors despite his failure at Enies Lobby, and it's strongly implied his strength has increased since he fought Luffy. But how strong is he?
Lucci's Abilities
In battle, Lucci relies on two main sources of power. The first is the Rokushiki style of martial arts, which entail six superhuman techniques every member of CP9 except Spandam have mastered. Lucci exceeds his comrades in his use of most of these techniques, and is the only member shown to use a secret seventh technique, the Rokuogan. He uses this technique by placing both fists against his opponent and emitting a massive shockwave. Lucci, like his colleague Kumadori, also is capable of enhancing these techniques using Seimei Kikan, or Life Return, a technique which allows him to control every last part of his body down to the hairs on his head.
In addition to his use of Rokushiki, Lucci ate the Neko Neko no Mi, Model: Leopard, a Zoan Devil Fruit which allows him to transform into a leopard. Most Zoan Devil Fruit users are able to access two forms: a hybrid form and a full beast form. Using Seimei Kikan, Lucci actually has two hybrid forms. Without Seimei Kikan, his hybrid form is incredibly bulky and emphasizes physical strength, and he grows significantly larger than his human form (though less drastically than some other Zoan users). When he uses Seimei Kikan, Lucci can shrink the muscle mass of his hybrid form to in exchange for speed.
Lucci has not been shown fighting after the time-skip except briefly in the Non-Canon One Piece: Film Gold, where he was shown to have mastered Armament Haki. That said, any other new abilities he may have acquired since his defeat are currently unknown.
How Strong is Lucci?
Lucci's power is easily the highest in CP9, with definitive proof thanks to a measurement called Doriki, which are essentially power levels for the members of the group. Lucci's Doriki of 4000 dwarfs that of the second-strongest members Kaku and Jabra, who possess Doriki of 2200 and 2180 respectively. This means that in terms of physical strength, Lucci's power is nearly double that of Kaku's.
It's much harder to gauge Lucci's strength in comparison to other fighters in the employ of the World Government. It is unknown how he ranks among the members of the secretive CP0, although he has been seen ordering a masked agent to capture Nico Robin, implying he isn't the lowest-ranked member. By Marine standards, it can be inferred that Lucci is at least at the level of a Vice Admiral, as few of them are capable of using more than a few Rokushiki techniques while Lucci has mastered all of them. The only definite exception to this rule is Luffy's grandfather Monkey D. Garp, who is arguably one of the strongest characters in One Piece and whose current rank is a result of refusing promotions.
Lucci is also almost certainly weaker than any of the current or former Marine Admirals, who all possess power comparable to some of the Yonko's strongest subordinates, let alone the current and former Fleet Admirals, Sakazuki and Sengoku. That holds true at least prior to the time skip. In Film Gold, Lucci fights evenly with Sabo, who had fought evenly in Dressrosa with the Admiral Issho, also known as Fujitora. Issho, however, left that battle willingly and it's unknown who would have won had it lasted longer, making it a difficult comparison.
It's especially hard to quantify Lucci's strength and that of other World Government fighters because unlike Pirates, they don't have bounties. What can be said is that Lucci remains a significant asset to his superiors and is likely to reappear in the near future as he was seen just outside Wano while Luffy and his allies fought the two of the Yonko, Kaido and Big Mom. CP0 as a whole also seems to have a major role in the upcoming story, and thus more may be learned about Lucci's strength post-time-skip in the near future.