Box mix
Reliable and fast. Swap the water for milk, add an extra egg yolk, and it closes most of the gap with scratch.
Every brownie recipe online claims to be the best one. They can't all be right, so I've made enough batches side by side to know that "best" depends on what texture you're actually after. This is the breakdown by camp, not by hype.
Fudgy and cakey aren't a quality scale, they're just different recipes. Figure out which end you're chasing before you touch a bowl.
Not every variable matters the same amount. This is roughly the order I'd fix things in if a batch came out wrong.
I've made all three enough times to have an honest opinion. None of them is wrong, they're just different trades.
Reliable and fast. Swap the water for milk, add an extra egg yolk, and it closes most of the gap with scratch.
Doctored box mix: brown butter instead of melted, a shot of espresso powder, chopped chocolate folded in. Best return on effort.
Full control over the fudgy-to-cakey dial and the best texture ceiling. Worth it when the occasion actually calls for it.
Most bad batches trace back to one of these, not a bad recipe.
This is the one thing people mess up more than anything else. It's not a special ingredient, it's technique.