Before we get into the nitty gritty—the most important thing to remember is the best whiskey for your cocktails is the one you like. In order to create a cocktail you will like you need to use ingredients you like! Traditionalists will say a true Old Fashioned should be made with rye since bourbon wasn’t developed or being made until decades after prohibition. Regardless, use what you like. For reference, bourbon will make for a mellow and slightly sweeter Old Fashioned. Rye will be a little “hotter” due to being high in alcohol and will add some spice. Use whichever you prefer, or do a mix of both for the best of both worlds.