This recipe is sublime. I went through an entire bottle of Ransom Old Tom much too quickly with these. Sadly I was out of Carpano Antica at the time, but Noilly Prat and Cinzano both worked brilliantly. I stuck to Regan's for the bitters most of the time, but I think there's room to play with the bitters if you're so inclined. The Old Time has such a rich flavor there are plenty of flavor notes you could highlight. Highly recommended.