*I will interpret Bane at his best to be RoT Bane*
Under normal circumstances, even without the orbalisk armour, I think Bane would defeat Obi-Wan in a lightsaber battle, and quite easily.
He has the greatest showing of speed in the mythos, moving completely faster than the eyes of powerful Force Users could see, and this while he was still a student with something like a year of training (he now has over ten). He's physically massive, described as a "seven foot mountain of muscle," and wields a form that emphasises strength, despite his incredible speed showings. His general showings with the Force, which would reflect on how effectively he would be able to enhance his physical capabilities with it, have entered the planetary scale in power and the subatomic scale in control, which makes him uniquely equipped with using the Force on a small scale as far as showings are concerned; a small scale such as his body. Physically, or through any Force related element of lightsaber combat, Obi-Wan essentially cannot hope to compete. Bane will be far quicker, far stronger, and will have greater reserves of energy. Obi-Wan has nothing to his name in this regard and by all available evidence is nothing exceptional. It's not close in this department.
Technically speaking, Obi-Wan kept in constant practise with his lightsaber ability and was highly renowned for his ability with Soresu. That being said Bane does demonstrate an incredible learning rate with the lightsaber (the kind we haven't ever seen elsewhere) when he becomes perfectly familiar with the hundreds of thousands of moves that make up Kas'im's knowledge of all seven forms of the lightsaber for the double bladed lightsaber in a matter of months. While he had been placing an emphasis on learning Kas'im's moves it would be unlikely that he wouldn't have placed a similar emphasis on his own, and given that Djem So was deemed the form he had the most talent in I would imagine his improvement rate with the form would have been even greater. And we do know that he kept in practise during RoT and sparred with Zannah in lightsaber combat, and this is over a ten year period, when the same Bane achieves feats on par with the greatest of masters in something like a matter of months. In terms of technical ability, I'd probably give Bane the edge again, though it's certainly closer in this department.
Now when we factor in the orbalisk armour, it becomes overkill. Not only does it provide considerable improvements to Bane's Force power, reserves of Force energy, adrenaline levels and stamina, which alone makes him a vastly better fighter on a physical/Force assisted plain than he already even was, but it also protects 90%~ of his body, meaning that Bane only has to really protect his head (everywhere else that the orbalisks don't fully cover are small enough and don't lead to vital areas that the orbalisk's healing rate will perfectly repair the damage in a matter of moments) and doesn't have to worry about the rest of his body. So essentially what we have here is a set of armour that takes one of the very elite combatants in the mythos, ups his physical and Force related characteristics substantially, and then provides perfect protection to 90%~ of his body. Bane only has to devote a minimal amount of his efforts, attention and stance on protecting himself, is able to devote far more to attacking offensively without exposing his vulnerabilities than any other combatant would be able to, is able to integrate his body into his style, both offensively and defencively, granting him an entirely unique style that Obi-Wan would be completely unprepared against (which as we all know on the offensive front has the effect of overloading an opponent's precognitive abilities), and we can now look back at what I was saying in the physical/Force assisted departments and amplify everthing substantially.
It's not close. To be honest this kind of thread is pretty worthless. Obi-Wan cannot hope to compete with the very elite of his own era (Yoda, Mace Windu, Count Dooku, Darth Sidious). I fail to see how his chances would be any better against the very elite of the entire history of the mythos, which someone like Bane most certainly is.