Base spirit: Everclear 190 or 151.
I generally dilute my Everclear 190 down to 151 with clear distilled water because of flavor extraction rates. If your base spirit is too strong, you will be extracting the full palate of flavor molecules, good and bad. I don’t know why it is that bad flavors are the ones that leech out from the substance later, but thank goodness they do!
A good quality vodka can also be used, but there is no real reason to bust your wallet with top rail stuff. I will say that a vodka infusion is generally a little more mellow out of the starting gate. If your infusions are too harsh every time, consider using high-quality vodka.
Primary bittering agents: gentian root, angelica root
Other bittering agents: orris root, cinchona bark (very bitter), orange peel, wormwood
Character flavors: star anise, dried juniper berries, new-growth fir sprouts, cinnamon stick, Madagascar vanilla bean, fresh garden-grown peppermint, rosemary, sage, orange and lemon peel, clove, allspice berries, ginger root, bay leaves, Earl Grey tea.
Mellowing agents: Toasted American or French oak – I prefer a lighter toast. Darker toasts will impart a very harsh smoky flavor (which isn’t a bad thing) that often build upon the bitter agents. If you prefer a bitter flavor that almost overpowers everything else, then the darker toast may be your thing.
Bittering agents: infuse in 190 or 151 proof neutral spirits for no more than 6 days
Character flavors: infuse in 190 or 151 proof neutral spirits for 1/2 day to 3 days.
Mellowing agents: infuse in 151 or 90 proof spirits for up to 6 months. Lower proof spirit will produce a much more mellow and rounded flavor.