Depends. Rag paper is good for editioning, but if you're printing with waterbase ink, it soaks up the body of the ink and flattens everything out. Also plan to print all layers at once to avoid paper swelling and shrinking due to changes in relative humidity in your shop, and the water in the ink.
Uncoated stock from French Paper, and Domtar's Cougar line are both solid picks for inexpensive, nice paper. 100# Cover is a great beginning / all-around weight that can be rolled for shipping.
Otherwise, I've done fine art editioning on Crane's Lettra line and on Coventry Rag, which is a generic cotton stock from Legion Paper.
Think about your artwork as well. Halftone and process work needs a smooth stock for best results. Otherwise the dots deform as they soak in. Cotton papers have more linters, which will clog the screen in some scenarios, especially if you cut down stock before printing.
Sorry for rambling, but there's a lot to learn about paper that is application specific. There's no magic bullet for all things.
Ever visit www.gigposters.com ?