Besides chocolate, what is the best flavor buttercream for a marble cake?

Strawberry. Next would be Lemon.

depends on the flavor/color of your marble. I'd coordinate them, i.e. pink/strawberry, yellow/lemon, brown/chocolate, orange/orange, white/vanilla etc.

was going to be choc. & vanilla, however, those flavors sound yummy. Icing will be a garnet/maroon color and a 16" to feed 90-100. (high school reunion)

If the icing is maroon/garnet, maybe a red velvet cake that is more garnet and an ivory or even a yellow marble would be very pretty. Have you seen the zebra cake technique; that would be more dramatic than the marble.
Be sure to post a picture.

Zebra!?!? Zebra?!?! This sounds too amazing Can you please tell me what I've been missing out on!

What about this it's one of my favorites. http://cakecentral.com/recipes/2047/crusting-cream-cheese-icing