I won’t say THE one exercise, but one I’ve been working on recently... permutation exercise I’ve seen described by Ari Capp, where you work your way up one fret at a time doing different fingerings. Like 1-2-3-4, then different permutations like 2-1-3-4, etc. Here’s where I found what I need to work on, alternating the plucking hand, index and middle finger, independent of the fretting fingers. It takes me some concentration to pluck with index finger if the middle finger is fretting. I don’t have problems during performances in articulation, but I see myself on the videos recordings using one finger more than I’d care to admit, so I’m working this exercise looking for improvement.