Price: Raspberry Pi Zero ($5) + Wi-Fi dongle ($3.49+)
Ranked in these QuestionsQuestion Ranking
Pro Large community
It's widely supported and you'll find a lot of resources on how to get whatever you want to do done on Raspberry Pi. It's as of 2014 still the most famous single-board computer (just check eBay).
Pro Good performance for the price
The Raspberry Pi Zero has a 1 GHz ARMv6 CPU and 512MB of RAM.
Pro Supports the Raspberry Pi Foundation
Raspberry Pi Foundation promotes studying computer science in schools.
Con Wi-Fi dongle takes up the only peripheral USB port
Pi Zero has 2 USB ports - one for peripherals, the other one for power. Since getting Wi-Fi on the Pi Zero requires a USB dongle that takes up the only peripheral USB port, a USB hub might be needed to attach keyboard and mouse.
Con No onboard stoarge
The device has a microSD slot and needs a card to act as storage.