My absolute favourite are Lindt's Lindor milk truffles, which are exquisitely melt in the mouth. These are a typical high street brand though, and so have more sugar than cocoa in them. In terms of quality, you want the proper artisan chocolate makers like Demarquette, Paual A Young, and Arisan du Chocolat. As a good medium between price and quality, I think Hotel Chocolat are the best, especially as they are first company to make white chocolate with a higher percentage of cocoa butter than sugar, and they have created a 'supermilk' range with more cacao and milk than sugar too. Their supermilk chocolate and carame flavoured chocolate are divine.
I would have to say Lindt's Lindor chocolates. They are the most delicious chocolates I have ever tasted that melt in the mouth. They have come out with a variety of new flavours to add to the favourites list.