MAJOR chocoholic here, so I'll pretty much eat anything, but I do have some favourites.
For more commercial stuff I don't think you can go past Lindt. The Lindor balls are the best! The few experiences I've had with American chocolate have been disappointing - it's usually overly sweet and just doesn't taste quite right.
There are quite a few "boutique" chocolate shops around in the city that have some of the best stuff. "Koko Blak" and "Max Brenner" are my personal favourites, they have awesome chocolate, but have cafes attached which serve some great stuff like liquid chocolate you drink out of an oil-burner type thing. I had this amazing hot chocolate at a south american cafe not too long ago. It was hot milk poured over good quality chocolate buttons, honey and chili paste! Another memorable one was in Vienna, where it came served in a metal goblet with all sorts of spices and cream on top.
As for flavourings, the darker the better, but I still love milk and white chocolate too. Anything infused/filled with anything orange, mint or coffee is also good in my book - particularly chocolate coated coffee beans.
Ok, so clearly I'm an addict. 😱