I'm a huge Kakuzu fan and Kishimoto rushing the Immortals arc due to pressure from his editors has caused a lot of people to underestimate Kakuzu when in fact he was immensely powerful. Definitely one of the top Akatsuki.
He has to be killed 5 times, he can seal shut any wound and reattach limbs, he can also steal body parts from other opponents in battle so even if you kill him twice you may still have to kill him five more times. His physiology also logically makes him immune to poison and genjutsu due to their specific effects on a normal human's body, Kakuzu is quite literally a mass of tendrils inside of a skin suit.
He has incredible intelligence, strength, speed, limited sensing, and experience. He was picking apart Shikamaru's plans and lecturing him on things he didn't even say out loud, he blitzed Kakashi twice, and his strength was sufficient to resist the Nibi's strike and to punch down a sealed iron gate. He was able to detect Team 10 from a distance and noted that he wasn't able to sense Kakashi's presence, which suggests limited sensing abilities. His experience also means that he has seen or experienced almost everything, which allows him to know what to do in most circumstances. Think of it like your first day on the job, there's so much to remember and you have no idea what you're doing... but a year later and you could probably do your job in your sleep. That's Kakuzu.
Jiongu also provides many other abilities:
Tendrils: The tendrils are strong enough to lift people off the ground, break bone, and restrain people. They are also very sharp and can easily pierce flesh. They can extend unknown lengths and can appear from any part of Kakuzu's body. In addition to all of this, he can control their trajectory so it's hard to escape them. Close-mid range combat is almost impossible against him.
Doton: Kakuzu can turn his body into adamant, a substance as hard as diamond. He can use this to tank almost any non-Raiton attack and to increase his striking power, making a punch from him like a sledgehammer.
He also showed the ability to travel quickly underground, as seen when he detached his hand with his heart and it slid through the ground to remove Shikamaru's trap.
Katon: Can carpet bomb the terrain in a wildfire, as well as shoot large blasts of fire.
Fuuton: Can create gigantic wrecking balls of condensed air, can also shoot large bursts of wind.
Raiton: Only displayed one Raiton jutsu, where he creates piercing electric spears powerful enough to singe Kakashi's hands even when they were being shielded by Raikiri. The databook also said that he can create as many as he wants, only using two because Shikamaru and Chouji were standing near each other.
Suiton: Never displayed, but even a generic water blast would make him more powerful.
Masked Ghosts: Kakuzu can detach his masks while giving them pseudo bodies comprised of the same dangerous tendrils that his own body consists of, they each also possess one element. They basically make Kakuzu a "Prototype Pein," especially since they provide him with shared vision. He controls the masks, even when his main body is dead he's still in control of them, and is obviously able to see through them or else they would just be running into things. The databook also says that with Jiongu he has no blind spots.
Finally there are his elemental combinations. We saw his powerful Katon+Fuuton combo, but he has the potential for other combinations. Katon+Suiton could create a boiling mist, Raiton+Suiton could create an electrified field, etc. He could also combine all of them at once for a supremely powerful attack.