My SO is getting into at-home mixology. For his birthday, I got him the new PDT book and I want to get some kind of slightly-off-the-beaten path liquor that he might be able to use to make cocktails. He loves all branches of the whisky family and has been experimenting with cocktails using rye, bourbon, etc. I thought about looking through the PDT book and getting him everything to make a particular cocktail, but I'm worried about buying something that can only be used for that cocktail. I thought about Cointreau, but he hates sweet things and therefore, Manhattans, so I don't know how much it would be used. I've heard that Cynar plays well with whisky, so thought about that. Any thoughts on other things that play well with whisky and might be used in a range of cocktails? Bonus points if it also plays well with gin, the other spirit most often used in our house.
Thanks in advance!