My favorite for the last year has been Swiss Miss Dark Chocolate kind.
I'd been making a homemade diet hot chocolate mix for hubby. It always clumped in the cup and was hard to mix as a result. I switched over to Hershey's Dark Cocoa Powder and voila! It mixes up w/o clumps and is as good as, if not better, than the Swiss Miss Dark Chocolate.
I use sugar in mine, dh gets the nasty Splenda in his.
Link to the post in my blog:
Another Healthy Hot Cocoa Mix