Manhattans are great with either Bourbon or Rye whiskey. It is very important that the Vermouth be fresh and good quality. Vermouth is best stored in the refrigerator. Many bars do not refrigerate their Vermouth. Like wine, it doesn't last forever. Vermouth is in fact, "Aromatized Wine", which means wine that has been flavored with herbs, spices, fruit or other natural flavorings. It is almost always fortified with additional alcohol, but the ABV (alcohol by volume) is much less than whiskey or other spirits. After a Vermouth bottle is opened it begins to oxidize. Ideally it should be consumed within a month of opening.
So, find a good quality bar and a bartender who knows his craft, and order a Manhattan. If you specify the type of whiskey you'd like the bartender to use, you'll look even cooler and sound like you know what you are talking about! Maker's Mark might be a good choice - most any bar will have it in stock and its a great Bourbon. Personally I prefer a Rye Manhattan, as I has a nice spice and a bit more complexity.
Or you can make your own!
Manhattan Cocktail Recipe:
-Place all ingredients in a mixing glass. Add enough ice to fill 2/3 of the mixing glass.
-Stir for about 20 seconds. The purpose of stirring is not just to mix the ingredients and chill the drink, but also to introduce a little water to the cocktail, ("dilution").
-Strain into your chill glass with a Harthorn Strainer.
-Garnish with a maraschino cherry, (Luxardo preferred) &/or orange twist.
-Put on some great music and say "Cheers" before your first sip!