We like Viaduc des Fauvettes

Vivien Badaut
Quite long routes (20 to 34 m) just along the RER B line.
77 routes fully equiped, from 5b to 7c .
Nice view at the top.
Topo (in french) : http://www.grimporama.com/topo-viaduc.pdf
Cillian Kearns
Funky little place! Very unique but the climbing itself isn't outstanding.

Guiz Augiat
Nice rope climbing place, easily accessible from Paris.