Andrew Chen makes an excellent point about metrics:
Metrics are merely a reflection of the product strategy that you have in place.
What you are trying to do should lead what you want to measure, not the other way around. It’s for this reason that the blanket questions and answers around “best” metrics are meaningless- the question is, what are you trying to do.
It sounds obvious when stated, but it's so easy to focus on measuring metrics without understanding why you're measuring them. That being said, because most startup's goal is to increase the bottom line, there are some fundamental metrics which, when used properly, will tend to appear in almost any sensible strategy.
But, at the end of the day, the relevant metrics depend on what you're trying to figure out, what hypothesis you're trying to test:
So ultimately, the important part is to figure out what you are trying to do and what the expected behavior is around it. Only once you have that should you then ask yourself how you’d validate and test it using metrics.