Ben at this moment. It might change though.
Ben held his own against perhaps one of the strongest suited-Vader incarnation. It was stated somewhere, that Vader is in his prime during "Rebels". In his duel with Ahsoka he really hates her and want her to be death (especially after she refuses his offer). Filoni confirmed that in one of his intervievs. But Vader's hatred for Kenobi is on another level. And that hate might made him even stronger the he normally is.
And - despite his age and being out of practice - Kenobi is still a master. He retained most of his skill. His knowledge and awareness might be even superior. He's pshycically rusted and weaker though.
A few days ago, I was nearly sure, that his superior knowledge and awareness made him harder to be catched off-guard. I'm not sure at this moment though.
Why? It's because of Vader. Once again, that guy disappointed me. He's powerful, hateful, he has a lot of agner withing him. His TK skills and overall command of the Force is great. But somehow, he's unable to use it as competently as Sidious or Dooku. Or even Kao Cen Darach. He's basing on brute strenght in nearly every fight. He Force pushed Ahoska once, but she found an opening in his Force-guard as well. He was winning that fight, but I would say that she gave him more trobule then she should.
In compression to him, Sidious, Darach, Dooku were a true artists of using the Force and sword combined.
That leads me to conclusion, that Vader - despite his skill and power in the Force - is, after Mustafar, far more clumsy and inferior fighter. I doubt that he would beat Dooku or Mace or even SoD Maul. And him being suggested - by some shit-source - as the best red blade user is laguhtable.
At this point, I believe that Force-guard depended more on awareness/concentration/balance. Dooku was able to catch Kenobi off-guard because he outmanuvered him in a duel short before and thrown him off-balance. Maul was able to catch Kenobi two times - at first time Kenobi had to split his attention between Maul and Savage, and second time Kenobi could be injured and seemed to be exhausted.
Even Maul was able to find an opening in Sidious' defense when he lowered his guard and gave most of his attention into a lightning's attack.
Maul on the other hand, at this moment seems to be broken and a bit rusty. His determination is as strong as ever though. He still can be superior to Kenobi and closer to Vader, but not in his current state. At this point, he's more like Ahsoka's level (which isn't as impressive like it seems).