As a fine chocolate connoisseur (ahem), I recommend sticking to 70% cacao minimum. 90% is usually disgusting. Always check the ingredients. A decent commercial international one is the Lindt 70% black. And I don't work for Lindt.
"Most clinical trials have used dark chocolate as the source of cocoa flavanols. In 2012, the distribution share of chocolate in the United States by favourite chocolate type was 57% milk chocolate, 35% dark chocolate, and 8% white chocolate [1]. We can therefore make the assumption that the majority of chocolate consumed in this sample was dark or milk, both containing cocoa flavanols to varying degrees"
But given this, another recent study comes in my mind: http://graphics8.nytimes.com/packages/pdf/oped40/Lustig.pdf (which just tells that sugar kills you with or without cacao)
> 90% is usually disgusting
Tsk tsk. Amateur.
If it's processed, it loses all its kick. Also I suspect processing reduces whatever benefits it may provide.
http://cdn2.blisstree.com/wp-content/uploads/2008/06/lindt-e...
I don't claim to be a specialist but I suspect the big difference co.es from cocoa fat vs solid percentage.
I now stick to milk chocolate not dark since I am allergic to chocolate the dark stuff makes me sneeze but milk chocolate seems to buffer the allergic effect.