which color can you notice the "marbling" of the leather the best?

When you say "marbling" are you referring to the white spaces between the veins that appeared on 2006 bags? If you are I'd say that the darker 2006 blues show the marbling the most. Anything dark colored, really, since marbling is white and it contrasts with the dark colors.
 
There are some colours that are overall much more marbled than other. LG is right, '06 had a lot of marbling but especially on Truffles and Cornflowers. This past S/S season, Bleu Glacier seemed to be very marbled, it was really pretty. This season seems to be thicker and smoother overall. I see less marbling 2005 and earlier and I know they were using different leather as the bags were not being produced in the same quantities. Hope this helps!
 
Blue Glacier is the first one that comes to mind...especially since I just bought one and can't wait for it to arrive!! ;) I have noticed in general that the "depth" of Balenciaga colors tends to even out and become more one-dimensional as a bag ages/gets use. Using conditioner on a bag speeds up that process. That's actually something that I hope does not happen w/my blue glacier. I have grown to really love the "mottled" look of that color (and at first I saw it as a downside).
 
My 06 emerald twiggy had a lot of marbling too. I didn't use it long enough for it to disappear though.

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My cornflower city has a lot of beautiful marbling and wrinkles. I used to dislike it but now I love it!

Sometimes greige can also have a very beautiful and subtle marbling effect...it tends to really look like marble! Very pretty!