Why the "right" substitute depends on the recipe
Eggs do several different jobs in baking — binding ingredients together, adding moisture, helping things rise, and contributing richness — and no single substitute replicates all of them equally well. A flax egg (ground flaxseed mixed with water, left to gel) is a strong all-purpose binder, good for dense bakes like muffins and quick breads, but won't help a cake rise the way a whipped egg white would. Aquafaba — the liquid from a can of chickpeas — whips up remarkably like egg white, making it the standout choice for meringues and macarons, but it's less useful as a plain binder in something like a brownie. Applesauce and mashed banana both add moisture well but bring their own flavor along, which matters if you're after a neutral-tasting result.
When egg substitutes work best — and when they don't
Substitutes generally perform best in recipes calling for one or two eggs in a dense, moisture-tolerant bake — muffins, quick breads, pancakes, brownies. They struggle more in recipes relying heavily on eggs for structure and rise — soufflés, angel food cake, or anything with more than 2-3 eggs, where the egg isn't just a minor binder but a structural backbone of the recipe.
FAQ
Can I mix substitutes for a recipe needing multiple eggs? Yes — some bakers use one flax egg plus applesauce for the rest, balancing binding and moisture rather than relying on a single substitute for everything.
Does a flax egg need to sit before using? Yes — mix 1 tablespoon ground flaxseed with 3 tablespoons water and let it sit about 5 minutes until it thickens to an egg-white-like gel before adding to your batter.