My mom makes a New York chocolate cheesecake and a chocolate torte called Queen Mother cake. They're both from Maida Heatter's Book of Great Chocolate Desserts, and they are amazing. I've been alternating them for my birthdays for most of my life. One year I'll ask for chocolate cheesecake, the next year Queen Mother cake. This year, for my thirtieth birthday, I'm thinking of asking her if she'll make both.
The only cake I've ever had that compared to those two cakes is a chocolate cake at the Rheinlander, a German restaurant in Portland, Oregon. I can't remember what it was called or what went into it, but it was exceptional.
After those, the next best cake is 86 Proof cake. That one's probably also from the Maida Heatter book. It's less fancy than chocolate cheese cake and queen mother cake, but it's a really good chocolate cake. And, if you generously drizzle raspberry sauce over it, well, it's pretty amazing.