OK anyone who's seen me around here knows I can't resist chiming in with a vote for the "tweaked" Fidge Brownie cake as posted here http://www.cakecentral.com/cake-decorating-ftopic-52587-30.html
(it's at the top of p3). Ignore the part about it cracking and doming, that was taken from the site where I found the recipe but I haven't had that problem. It makes great cupcakes too.
I've made the double chocolate layer cake, the darn good chocolate cake, the hershey's perfectly chocolate cake, and who knows which others, and IMO this one is the darkest, richest, most chocolatey I've ever had and is my 100% go-to chocolate cake. It has turned me into a chocolate cake snob: at this point I've yet to find a bakery chocolate cake that I like better, so I don't even bother ordering chocolate cake when I'm out. (Fancy restaurants and soufflés not included.)
I haven't tried the chocolate stout cake tania9 posted but have seen others here recommend it. At this point though, the FBC is SO good I have stopped looking for better, I just don't see how it can get any better
. As always though, we all have different tastes! Interestingly enough even the original reviewers of that cake couldn't come to a consensus about which they preferred, the FBC or the DCL cake. So try them both!