RPi Family:

RPi 3:

• Best

• On backorder

RPi 2:

• Available

RPi Zero:

• Small
• Cheap ($5 / Pi)

• Not in stock
• No built-in Ethernet (http://raspberrypi.stackexchange.com/questions/38622/whats-the-cheapest-way-to-get-network-connectivity-to-the-pi-zero)