Pure vanilla extract

I don't think that vanilla is always necessary. Its my opinion that we rely on vanilla and cinnamon way too much in American baking. That being said, I think a side by side tasting would prove that vanilla is absolutely detectable in certain baked goods. For instance, I think vanilla is a very crucial flavor in chocolate chip cookies. Brownies...eh....it's subjective really. The most prominent flavors in my brownies are chocolate, salt, caramel and coffee. To me, vanilla isn't crucial, but I add it because I'm a baking sheep ;) I still think there are more important flavors in baking that are often overlooked (salt, sufficient color, acid, etc.).

For brownies specifically, I find that the addition of vanilla rounds out the taste; there are brownie recipes without vanilla, however, I often add it for the complexity it brings to the final product.