If you're going to have any hope of surviving One Piece's Grand Line -- let alone becoming the Pirate King -- you need to have a flexible set of combat abilities and skills. And that doesn't mean just being able to stretch your limbs the length of a galleon.
The Straw Hat Pirates are a group of talented individuals, each with strengths and weaknesses. Let's try and discern which of these 10 pirates has the most adaptable fighting style that makes them the most capable, no matter the situation. Keep in mind, this is not about who is the strongest, but whose abilities are most versatile and applicable in any given fight.
Let's start with Nami and her Climtact. With her weather manipulation weapon, Nami can manifest billowing winds and call upon thunderstorms, even indoors. She's clever enough to be able to swap places with a mirage duplicate of herself when her enemy's distracted, and with Zeus now residing in her weapon, she can deal physical damage by having the homie manipulate the Climtact's shape. Nami's abilities are best suited to taking out swarms of enemies at once, however -- the navigator has more trouble against a single, more resilient enemy.
On the flip side, Robin's devil fruit abilities are a serious contender for most adaptable combative ability. As long as her opponent has a spine or neck to break, Robin will always have a clear way to victory with her Flower-Flower fruit abilities. The only time the rug has been pulled out from under her in One Piece is with Moria, who was able to swap places with his shadow doppelganger.
Luffy may come as the most obvious choice, and with good reason, as he's fought and defeated countless powerful foes across his journey. The way his Gum-Gum fruit powers have developed and evolved alongside him, allowing for some creative and powerful techniques and abilities, is beyond commendable. Luffy's haki ability is nothing to scoff at either -- even in Wano, he's found ways to advance his skill level with this power. However, like any devil fruit user, Luffy has his weaknesses. Despite his tenacity and dedication, no amount of training will negate his weakness to seawater or sea prism stone.
Though the likes of Sanji's Raid Suit, Brook's devil fruit powers, and Franky's Battle Frankies are all indeed powerful, they each come with respective drawbacks in certain situations. Therefore, the highest contenders for most versatile combative abilities in the Straw Hats are probably Zoro and Jimbei. Neither possesses devil fruits, so they have no outward weaknesses to speak of. Both also have tremendous aptitude with Busoshoku Haki; Jimbei was able to fight Ace for five whole days when he first met the Flame-Flame Fruit user. He was also able to coat his entire body in Haki, a feat only a rare few possess. On the other hand, Zoro can completely coat all three of his swords in Busoshoku Haki at once and was even able to cut the mountain-sized Pica in half, despite his Stone-Stone fruit powers.
Their physical abilities are also well-suited for One Piece's life on the high sea. On top of his excellent haki ability, Jimbei is a Fishman and a master in Fishman Karate, making him ready to fight any type of devil fruit user. He can even capsize ships by manipulating the ocean waves. Zoro is also constantly testing the limits of what he can cut. Whether it be man, machine, or even nature, Zoro has never faltered when it comes to testing his steel against anything that stands in his way.
Put simply, Zoro and Jimbei have the broadest techniques applicable to any situation the Straw Hats encounter in One Piece. Of the two, you might say Zoro edges it a bit since Jimbei is only at his peak underwater, but you'd be sorely mistaken if you think Jimbei can't obtain victory on dry land too -- his victory over Who's-Who is proof of that.