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.