Green & Black's Organic chocolate with currants and hazel nuts. Costs stupid money (about £1.50 a bar from Sainsbury's IIRC) but it's the dog's danglies. Same goes for any of the others (the dark choc is my other favourite).
Lindt dark chocolate with 70% cocoa is stunning as well....
For an easily-obtainable one, it'd have to be the raisin and biscuit Yorkie.

Note that these are readily available. The best ever was some stuff I got from a tiny little shop on the Franco-Belgian border on the way back from a trip to Germany. Unbe-****ing-lievable, it was...
