It really depends on what you are doing, and what your goal is.
For my web app, I track the number of free and paid accounts in the last 1,7,30,60,90 days. I also track how many users have signed in in the same timeframe, as well as how 'much' they are using the product.
Of course, this is on top of the standard google analytics stuff, including conversion ratio.
For web apps... I recommend you track AARRR aka startup metrics for pirates. It stands for Acquisition, Activation, Registration, Referral, Revenue.<p><a href="http://500hats.typepad.com/500blogs/2007/09/startup-metrics.html" rel="nofollow">http://500hats.typepad.com/500blogs/2007/09/startup-metrics....</a>
How many users/customers we talk to each week. The higher that number gets, the more momentum we feel and the less we feel like we are lost in the woods.