A precise bathroom scale. I am trying to get my weight under control. I know I should slowly take it off with new healthy eating habits. And since I have also been building work out habits as I transition from rehabilitation physio to functional physiotherapy I won't necessarily see weight loss since muscle weights more. But I would love to see some progress...clear numerical evidence. Clothes being more comfortable again is just not as tangible.
I personally find measuring to be the most informative way to track because it's not one number that's heavily influenced by water levels, which fluctuate wildly, especially in women.
Obviously measurements are impacted by water retention, but not every number the same way, and the relative changes in the numbers gives a lot of insight into whether it's water or fat or muscle that's changing.
For example, my hip measurement goes up more relative to my thigh measurement if I'm retaining water because I retain so much in my lower abdomen.
So if the thigh goes up at the same rate or more than the hips, I can be certain that there's fat gain, unless I have radically upped my weight training in my thigh muscles and can see/feel that there's more muscle instead, but that would be pretty obvious.
It really helps me track composition much, much better than the scale does, and composition is really what matters.