The most popular and flexible arduino board is arduino UNO R3 board. The Arduino UNO R3 is a new board that has some additional features. This board uses the Atmega16U2 instead of 8U2 and allows faster transfer rate & memory. The Arduino R3 also adds SDA & SCL pins which are next to the AREF.
Priyanshu, you must know about lilypad arduino and arduino mega. Lilypad arduino is designed to integrate easily into an e-textiles. This Arduino board consist of an Atmega 328 to keep it as a small minimum external component is required. The power supply of this board ranges from 2V to 5V and offers large pin-out hole that makes it easy to connect. Whereas, arduino mega, is a type of microcontroller and is based on the ATmega2560. It consists of a total of 54 digital input/output pins. There are like crystal oscillator of 16 MHz, and the board has USB connection, RESET pin, ICSP header, and a power jack.
Preetham and niranjan have already mentioned about the popular boards. Here I would like to mention about arduino red board. This Arduino board is programmed by using the USB cable of mini-B and Arduino IDE software. The Arduino red board is constant because of USB and FTDI chips and are flat on the back.