Though this is completely unfounded, I kinda think that Isshin was the guy Zaraki beat to become captain. Again, these are all completely unfounded speculations but take a listen to my reasons.
1. Isshin (presumably) was a captain. Now he isn't. Why is that? Do people just retire from being a captain? Genryuusai is kind of proof against that but who knows. And I'm pretty sure that Zaraki was the most recent to be admitted to a captain status.
2. Zaraki's coat is tore up, Isshin has rags of a coat hangin around him.
3. Zaraki and Ichigo have a fairly important role together. I think that the emphasis put on their relationship and Zaraki's infatuation with Ichigo might stem from some kinda feeling of Deja Vu from fighting Isshin, who i would expect was a potent fighter back when they fought.
As for counter arguments. I realize that in order to become a captain, you need to kill the previous captain, but the way i see it, Isshin is buds with Urahara, who can of course do anything he wants to cause hes a freaking genius. So he probably set something up. I don't really know the time sequence of everything in Bleach so i could be totally off and Zaraki could have become captain long before Isshin could have even existed. Again, i dont know. But speculate, id be interested to see what other people think.