This depends entirely on the stage you're at with the business. Although some people have mentioned churn, no one has mentioned any measurements around engagement -- i.e. "Is anyone using this product at all, and in ways that mean they'll keep using it?"
In the early stages that's what I would focus on -- a measure of engagement. If you prove engagement (or what I describe as "Stickiness") you can move to the next stage, and now you're interested in seeing if engaged users stick around long enough. That's where LTV comes in. But there's no point measuring LTV or CAC at the beginning--you just started, and you don't know if your product is doing what it needs to in the first place.
If you get solid engagement with a small group, you might then say, "Now I want to bring more people in the top of the funnel and see how they respond" and care about sign-up conversions, while also looking at free to paid conversion from the early adopters (if you're going freemium).
If conversion looks good, and you can map out a scalable business based on that simple number, then you look at churn and LTV. "People are engaged and using the product, converting to paid and then dropping off..."
My2Cents.
P.S. This answer is heavily influenced by Ben Yoskovitz's writing :)
