Like I said, I'm with Zaraki on this one. The argument I've heard for Byakuya is that if you look at what happened in Soul Society when Ichigo went to save Rukia, non-bankai Ichigo was barely able to defeat Zaraki, but using Bankai, he wasn't able to defeat Byakuya. My guess is that Zaraki underestimated Ichigo since he couldn't even cut Zaraki at the beginning. If you look at what's happened in the manga
*spoilers if you haven't read it*
Vaizard Ichigo couldn't beat Nnoitra. Granted, he was worn out from his battle with Grimmjow, but he couldn't even scratch Nnoitra. Then Zaraki shows up and easily destroys Nnoitra in practically no time at all.