Earth is the best. It creates the best defense, and it creates a nearly unblockable offense. It can also be combined with Fire (to make lava), Wind (to make sand), and Water (to make mud).
Earth can block Fire, Wind, Water, and other Earth attacks. The only thing that can destroy Earth is Lightning, and Lightning is the rarest element, so not many shinobi have it.
The same can't be said for any other element.