Out of the dozen bottles of bitters I own, my favorite, and the one that I hoard with Gollum-like tenacity, is the Whiskey Barrel Aged Bitters from the good folks at Fee Bros. in Rochester, New York. I've written about these products previously, and even did a taste test using all of my bitters at the time with the Manhattan cocktail.
These bitters are similar to the Fee Bros. Old Fashion, but are aged in the charred barrels that were used to make Tennessee whiskey. For me this is fascinating: empty barrels from my state go to upstate New York, where they are filled with bitters, and then the resulting product is shipped back to me in small bottles. The flavor is certainly more complex and smoother than the regular Old Fashion, and it lends a distinguished, smoky element to the proper cocktail. Obviously I had to test it out with another Manhattan, this time with my friend Paul splitting the whiskey portion between Bourbon and Rye. Absolutely amazing. And if you want something really cool, make one with just Rye, Red Vermouth, and the bitters. It's like breathing in a whole library worth of antique books. One sip and you're thrown back in time 70 years, and you desperately want a cigar.
You can order a sampler of 6 bitters or the Whiskey Barrel Aged Bitters from my Amazon store.
Note: I received this bottle of bitters as a sample from Fee Bros.