Euler
Back in March, I posted a <A HREF="http://blog.stereophile.com/wesphillips//index4.html">link</A> to a <I>Physics Web</I> <A HREF="http://physicsweb.org/articles/world/20/3/3/1">article</A> on iconic equations, which quoted Gauss' assertion that if Euler's formula wasn't immediately obvious, the reader probably has no chance at being a first-class mathematician.