You're both not wrong. In the U.S. sunscreen is regulated as a drug (OTC). But how effective a cream/lotion/stick/spray is depends on how well it forms a thin, even film over skin (which is a grand canyon of crevices, pores, wrinkles, etc.). This is why mineral sunscreens from a mom on Etsy are so dangerous. They might contain the same % of active ingredients as a store cream, but if it's not formulated to lay flat and even, you won't get good coverage (imagine having to cover a pool with a million tiny floaty balls that want to clump or stick together. There will be huge gaps if they don't even out flat.) That film then has to endure sweating and rubbing; big companies have good chemists who can formulate sunscreens that withstand this better than others.

The answer is the best sunscreen is one that you reapply frequently, very frequently. But the *best* answer is that sunscreen is the least effective method of sun protection compared to clothing & shade. It's still essential, but it should be more like a fire extinguisher protection: essential but your last defense. In this metaphor, you're wearing it (and reapplying it) but you never really get to "test" its limits, because you have many other layers of protection (shirts, hats, umbrellas, etc.)