Is dark chocolate the most suitable type for fondue/choc fountain?

I've tried morinaga bitter choc and hershey's dark choc to melt in a bowl over hot/boiling water. Both went well except cadbury milk chocolate which did not melt to the right consistency, more of a thick cream's texture. Is it that only dark chocolate works or is it with the brand? I need a good suggestion of a worthy brand n type of choc. Thank you very much!