I'm going to recommend Wyeast's Thames Valley II for a chocolate stout with the caveat that I've never tried that yeast in a chocolate stout. I have, however, used it in a RIS and the smell of the chocolatey dark malts/grains and the esters from the yeast smelled about what I imagine heaven to smell like. It was sensational. So judging from that it's worth trying, and I'll definitely be doing it soon.