I'd say WB. He fought evenly with Roger many times apparently and was Garp gave Roger trouble and was able to "corner" him, but as far as I know nobody's said he was able to fight on par with him, so I'm guessing it was more that he chased him down and then Roger would have to fight a little to escape. More like Fujitora at Dressrosa or Smoker at Arabasta, rather than the proper fights we see Luffy in vs other pirates. There is still the possibility he was strong enough to properly rival Roger though and just have let Roger get away because of other distractions. I just don't think thats the case, although it'll be cool if it is.
Also, just going by how Akainu vs old, weakened WB went, I think the strongest marine was probably always below the WB, assuming they're a similar amount away from their lifetime peak. So young (pre-skip) Akainu is weaker than old WB and peak (post-skip) Akainu will be weaker than WB to a similar degree. That's just guessing how the story will be written though and not based on in-story evidence. Garp could be a little OP though, since he's Luffy's grandfather and has the will of D (not that that helped Robin's friend Jaguar D Saul).