I vote the Procope, I think the atmosphere is incredible and just seems more conducive to reading (not to mention much more comfortable!! than squatting on the ground or something to that effect) and discussing. The others will have more people milling about (I was nearly stepped on several times when I stopped to reflect for a while in the Conciergerie...) 2nd choice would be the Palais Royale, for the same reason that it's more open and comfortable - and I just like the idea of meeting in places where these people would have actually met, whereas glorious as the Pantheon is, it doesn't seem like a place for reading (for me, anyway, I was too busy bawling).