If I could have only one, in ride metric available to me, it would be cadence. I'm am precisely one of those riders who will do one of these things in the absence of a cadence meter:
1) Pedal way too slow if I'm not paying attention.
2) Over compensate and pedal a fair bit too fast if I am paying attention.
Others have done a great job of explaining the benefits of well managed cadence. To that, I'll just add my overall impression of the difference that proper cadence has made in my own riding:
3) I'm faster.
4) I somehow feel even faster than I actually am, which is wonderful.
5) A lot less of my cycling feels grueling .
6) I think that my shifting might actually be a bit smoother.
My ride computer is a $100 thing with GPS that attaches with rubber O-rings. My cadence sensor is a wahoo that zip ties to my crank arm. I can move this setup from bike to bike in ten minutes which I consider to be a substantial boon. I don't care about climate change quite enough (yet) to conserve zip ties.