Which trader has the most caps ?

There's not really any trader that has more than others, it's pretty much a random thing and changes every 3 days when their inventory respawns. However, if you invest in the caravans they'll end up having the most caps of anyone. You have to go to Canterbury Commons in order to do that.

Yup. The caravan trader you've invested the most in will have the most money. Also, what I usually do is, to increase how much they have, I all their stimpacks (as we all know, you can never have too many stimpacks ) and other chems, until they have enough to buy all my loot.

I think the devs did a good thing by making it that way, it actually encourages trading, like you would do out in the Wastes. It's kinda strange how in Oblivion the traders money never changes, they fixed that here.

I've found that the best repairman has the most caps, since they make a killing fixing up my weapons. Especially when I'm using the A3-21 rifle: awesome gun, but horribly expensive to keep in shape until the Enclave shows up.

It usually happens in my game that once I've fully invested in the caravans, Crazy Wolfgang will have a slight edge over the others when it comes to having the largest supply of caps.

Not to mention the highest Repair skill.

Typically, I invest 1300 caps in the caravans: 200 with all of them, and an extra 500 with Wolfie. That way, I have three very competent roving repairmen, and one outstanding one.