My rule of thumb for cocktails is - they were created to mask the flavors of shitty base liquors so this is what should be used. The best old fashioned comes from jim beam white label. Unless you use some really strong medicinal smoked whiskeys you probably won't be able to pick up much of the delicate flavors from the good stuff with so much syrup and bitters in it. and if you want to add complexity - just make a more complex syrup.

I'm not advocating to use limited release bourbons, but when 90% of your drink is the base spirit, you'll get a lot of value out of using a traditional top shelf bourbon vs well like Jim. Try a $40 bottle like Eagle Rare or Angel's Envy. There's not much syrup in a good old fashioned -- go with 2oz bourbon, a teaspoon of simple or good maple syrup, and a couple dashes of Angostura. Maybe a dash of black walnut bitters, too. The higher quality whiskey should be apparent.

When I treat myself to an expensive bottle as a treat, I go against conventional wisdom and use it at least once to make my favourite cocktails. While I agree that a simple bottle is best suited for cocktails, I can't remember an expensive base liquor ever spoiling a cocktail. For some, this is a waste because the premium taste takes a back seat to the cocktails, but often the result is better due to the subtle flavour profile of higher-quality spirits.

To be fair Jim Beam is not that shitty. It’s cheap but there are way worse bourbons.

you can make cocktails with subtler adjuncts just fine. Tea bases are a bunch of fun

I recommend a high proof (bottled in bond or cask strength) rye in most whiskey drinks

Wasn‘t it Woodford Reserve? I think I never used Jim Beam for an old-fashioned.