What, just one? No way!
Ok, if there were only one brush I could invest it, it would be the famous Mac 217.
This one did totally change the way I was applying makeup. You can pretty much do a full eye look with it if you have pigmented enough eyeshadows (because obviously this brush won't pack on color so much since it's so fluffy). I also use it to apply cream eyeshadows as it blends them perfectly.
However I can't resist taking this opportunity to tell you about another brush I like and that is, in my opinion, totally underrated (I've even seen Mac makeup artists who didn't know what to do with it!), it's the Mac 227.
This one is a big large fat dense eyebrush, but it's rather soft. It's perfect to apply a wash of color in virtually one swipe as it's so large, but what I like to use it for is applying my browbone higlighter, as this brush will perfectly blend the edge with the crease color. I also use it as my "eraser" to shape and blend the outer limits of my eye makeup. And finally, it's fantastic to apply powder hilighters on the cheeks, even those in lose form.