My go to for a regular Old Fashioned is Rittenhouse Rye.

For a bourbon Old Fashioned, I would go with a mid-tier bourbon such as Four Roses Small Batch, Elijah Craig Small Batch, New Riff Bottled in Bond, or for a more budget option Evan Williams Bottled in Bond or Old Grandad Bottled in Bond. In a cocktail where the spirit is so centered, the added flavor the mid-tier brings helps elevate the drink.

But you can also add spice and warmth by adding Angostura bitters instead of, or in addition to, the orange bitters. Bourbon + orange bitters is all about mellow, so turn to a rye whiskey (Rittenhouse) or high-rye bourbon (New Riff, Old Grandad) and Angostura bitters for warmth and spice.