Mine is cash runway, meaning how many months I can keep going at the current burn. Everything else, like MRR, tasks and pipeline, matters less if that number is shrinking.
I check it first thing because it decides what I can do that day: whether I can spend on something, whether I should slow down, whether I have time to experiment. I look at it in Trenston's daily briefing, right next to anything that changed overnight.
Curious what yours is. Cash, MRR, signups, something else?