Retention.

Retention means you've built something valuable and differentiated. Of the AARRRs, it is the closest approximation to Product Market fit.

Once you've nailed Retention:
- Acquisition is easier: you have a product people want)
- Activation is easier: tell people what it is that others like about the product
- Referral is easier: every time users come back, you have another chance to get them to refer you
- Revenue is easier: you have something consumers value and want to use. Either scale and sell ads or sell premium features.

Retention obviously isn't perfect: you could have found the only 100 people who care about your product or maybe it takes 15 minutes to convince people to even try it, but having fantastic retention is certainly a great foundation for building a huge company and a brand.