This is a personal question, different people have different taste. And it changes over time too. I can't like anything that is tooth-shattering sweet but can appreciate bitter flavor now.

My answer to this is always a Negroni. I cannot apprehend it, and I like cocktails. The Negroni is a cup of bitter sugar syrup and I cannot taste whatever it is that people like in it. It's like someone drank Campari and said "not sweet enough" what the heck?

Tequila, scotch, bourbon, all those I drink "lengthened" with some soda water to appreciate them and can taste the delicious flavors once they are diluted. Gin is good with tonic & lime. But the very strong, oversweet stirred drinks I don't enjoy and Negroni is the most beloved of them.