If you're in the southwest part of the country or see these around, they're harder to find and not just chocolate (more so candy bar/nougat than just chocolate), but Bottle Logic Study Break or Fate King Size Candy Bar Stout would be my top choices. Also southwest, Belching Beaver Chocolate Peanut Butter Stout.
Xocoveza is an excellent choice (and it's year round here
)... and another that comes to mind has also been suggested, Sam Smith Organic Chocolate.
Southern Tier's dessert beers are all excellent... Choklate, Choklate Orange, and they now have a Chocolate Milkshake Stout.
Epic Big Bad Baptist Chocolate Rapture... haven't tried it, but it'll get opened sooner than later and I'm sure it's great as well.
I'm partial to Yeti, too... Great Divide Chocolate Oak Aged Yeti, if it's still around.