Best Arduino Board/Kit for beginners who would like to hack?

Hello. I am new to hacking and it seems fun to me. What is the best Arduino board for me? (easiest to understand and use.)

The Uno.

What is your definition of 'hacking'?

At school our favourite beginner board is the Leonardo. We did away with all UNO clones we had before. Leonardo has some great extra’s without being more complicated and fully UNO compatible (as long as tutorials use the right pins: SDA/SCL and ICSP, not A4,A5 for I2C or 10-13 for SPI).
If you need a single reason to get a Leo instead of an UNO: dual serial ports. Which means that GPS/MIDI/GSM etc work straight away. Another benefit: there’s also analog pins on the other side. Which allows us to easily add 2 thumb-sticks with a band cable on both sides without the need for prototype boards. Micro-Pong is one of those nice beginner projects.