NMN (nicotinamide mononucleotide) and even just plain nicotinamide. Taking a large dose of nicotinamide (3g/day divided into 3 spaced-out doses) has resolved several relatively minor issues which I suspected were caused by some genetically caused mitochondrial deficiency. For example, I had some skin problems, including inflammation along the hairline, acne-like lesions on the neck and so-called "histamine intolerance" (inflammation-like red splotches). After a few weeks of taking nicotinamide all of these started clearing up and after a few months disappeared completely. Also I have higher and more even energy levels throughout the day.
One specific change that points at improved mitochondrial function is that I used to get shivering cold very easily, especially when wet. Not anymore! Just yesterday while walking for exercise I got surprised by a downpour and continued my walk soaking wet without any discomfort.