As an evolution of the first Arduino boards (first the Arduino USB, then the NG, next the Diecimila, the Duemilanove, and finally the Uno), the Arduino Uno has been refined countless times while retaining good shield compatibility and a stable code base.
Widely used; code examples are easy to find and comprehensive
Since surpassed for high-performance usage by other, form-factor compatible platforms such as the Netduino and LeafLabs Maple
Getting used to programming on the confines of an Arduino steeps one's understanding with the fundamentals of embedded program design. SIGBots therefore recommends that newcomers to embedded systems programming (particularly those on robot programming or electronics subteams) take their first steps on an Arduino Uno. The comprehensive array of code samples also makes it a good choice for personal experimentation.
The Arduino Mega is an expanded version of the Arduino meant for memory or IO-hungry projects. It costs twice as much as a regular Arduino and is nearly two times larger, but fails to make significant progress in CPU power.