The answer to the OP is: rye. If you want to use bourbon, ideally use a high rye bourbon like Bulleit, Four Roses, or Buffalo trace. But a straight rye whiskey will provide the best base spirit for the sugar and bitters to accentuate. Bourbon is a very sweet spirit, leading to an over sweet cocktail. Your mileage may vary, but rye balances out the simple syrup and for whatever reason, angostura bitters plays with straight rye better than bourbon. I've done numerous flights with coworkers (work in the spirits industry, used to work at a Kentucky Bourbon Distillery) and rye always, always comes out on top.
suggested ryes: Rittenhouse, Bulleit Rye, George Dickel Rye.
That's what I get for relying upon Downton Abbey as a source of historical information.
The old fashioned is named so because it was an "old fashioned cocktail" just bitters, sugar, and spirit. When consumerism and international trade really took off in the 1880-1890s, suddenly bartenders had more than whatever local spirit was available to make drinks with and began adding "cordials" from Great Britain, amaros from Italy, and fortified wines from France and Spain. Some people didn't like the new, inventive cocktails and began calling for an "old fashioned cocktail."