As a cheaper alternative to 100% dark chocolate, I drink unsweetened* cocoa made as follows:

1. Fill kettle, start boiling water
2. Add 2 teaspoons cocoa powder to mug, stir in a small trickle of cold water to make a paste
3. Stir in boiling water, filling mug about 2/3rd full
4. Top up with cold milk (*milk contains sugars so this isn't technically zero sugar. You could skip the milk if you think it matters, at the cost of worse taste).

Best tasting cocoa powder I've found in the UK is Cadbury Bournville. Some supposedly premium brands (eg. Green & Blacks) taste much worse, bland with a slight burnt taste. Color seems to be a good indication of taste - lighter color generally tastes better.