Paul Clark, writing under the "PROOF" banner of the New York Times has an outstanding post about the revival of the American Cocktail Culture. It's inspiring, actually. One is left, upon reading it, with a desire to set out on the road looking for America's finest builder of Manhattans or the simple artisan behind the bar dedicated to mixing up something authentic and old and found in a book on mixing from the first decade of the last century.
The beauty of the Manhattan is that it can be used as the base for showing off a bartender's own personal style. To that point, the traditional Manhattan consists of some form of Whiskey, sweet vermouth and bitters. However, I've noticed a number of bartenders forgoing the bitters all together. I prefer the bitters included.
What I've noticed is that the way the customer tends to personalize their Manhattan boils down to the type of whiskey they order with it. I prefer Makers Mark. A lovely friend of mine prefers Crown Royal. But what's most important for me is that the Manhattan not come with a froth on top. That means stir it with ice, but don't shake it. I want my Manhattan to be clear, like my mind before it is Manhattanized.