You should use dark chocolate with at least 70% cocoa solids. Any lower and the purity isn't there; if you use chocolate with, say 85% cocoa solids, then the result might be too bitter, plus you could have melting problems. I personally don't ever use 'cooking' chocolate and wouldn't recommend it.
Use Valrhona...
that's what the Four Season's Hotel, Mandarin Hotel, top restaurants uses.
I agree with Thames advice...dark chocolate with at least 70% cocoa.
Anything over 85% will be too bitter.
I would definitely recommend Valrhona - much better than any other brand. Expensive but worth the money. I use it with my molten chocolate cake recipe and every time it is a real success.