Carbon
Carbon is the Devil's element, with six protons, six neutrons (in the most common isotope) and six electrons, four of which are valence electrons.
Carbon loves to bond, and so is also the patron saint element of relationship gurus and the whore of the periodic table. Although carbon forms stronger bonds with fluorine and oxygen than with itself, carbon-carbon bonds are so strong[1] that the two best-defined forms of elemental carbon are resistant to chemical attack and remain solid to very high temperatures.
The reason carbon bonds so readily and in such variety (slut!) is that its second electron shell is half full[2] — 4 out of 8 energy levels are filled. As such carbon is very happy to share electrons with almost any reasonably friendly nearby atom — even other carbons.
Pure carbon forms several different compounds in different ways. Diamond is a three dimensional lattice that due to its stable bonds is very hard. Very, very hard. And pretty when cut well. Graphite forms 2-dimensional sheets that are very loosely bonded to each other, making it very slippery. Synthetic buckyballs are geodesic domes formed of 60 carbon atoms; buckytubes are similar but form long hollow fibers.
The study of how carbon interacts with other elements is called "organic chemistry" and smells really bad.
Hydrocarbons are a convenient way to store chemical energy for later use via oxidation.