Arduino UNO is the best!! !=D
I would say Arduino Duemilanove as you can get it for real cheap on ebay and its more sort of hackable...But get the Arduino Duemilanove AT-MEGA 360!But it depends on what you want more ports?? more power supply?? better functionality?? Long life??But if you compare the duemilanove with the Mega then mega is way better!
steveastrouk's answer not good enough?
THREE times that question's been asked in 48 hourshttps://www.instructables.com/answers/Arduino-Boards-Which-is-the-best/