Arduino UNO is the best platform for the beginners. It is a multipurpose board and it’s using controller and can be powered directly from the USB battery or via an AC-to DC adapter which uses the ATmega 328 chip as the controller .If you are using an adapter you have to use a 9V or 12V wall-wart.
The UNO features 14 digital input ND THE output pins, out of which 6 of get used as pulse width modulation or(PWM) outputs. It also features the 6 analog inputs as well as TX/RX (serial data) pins. The analog pins gives resolution of total of 10 bits, which means getting 1024 different values when one reads them. This is quite a high resolution, and it is accurate by 10 times than MIDI. Each of the pins can draw atleast a maximum of 40mA current.
The board has 32 KB of flash memory, 2KB of the SRAM and 1KB of EEPROM. Flash memory is the program that you have written in stored, SRAM is working memory or scratchpad and EEPROM is the storage for variables.
So the best arduino board is the Arduino Uno which can be used by the beginners as well as pro for smaller projects or know how purpose.
Leonardo Arduino is a multipurpose microcontroller board which is based on ATmega32u4 (datasheet). It has total 20 output/input pins, crystal oscillator of 16 MHZ, a power jack, a micro usb connection, an ICSP header, and including a reset button.For supporting a microcontroller there is every input in it.You can simply connect it to a computer with a usb cable or power it with an ac to dc adapter or a batterry to get it started. The Leonardo differs from all preceding boards in that the ATmega32u4 has built-in USB communication,eliminates the need for secondary processor. It allows the Leonardo to get connected to computer as a mouse and keyboard, in addition to a virtual (CDC) serial / COM port. It also has other implications for the behaviour of the board; these are detailed on the getting started page. Leonardo is much advance in terms of features in all the earlier boards by arduino.The ATmega32u4 has a built in USB communication, which eliminates the need for secondary processor which leonardo appears to connected computer as mouse and keyboard and adds a virtual (cdc) COM or serial port. It has other implications for behavior board.