Uno and Mega still have the most documentation. I would say, if you're just starting up, still pick one of these depending on your application. I do believe Uno is the most widely supported for beginners.
Even cheaper on eBay, BangGood, DealExtreme. I would stick with ones that have the Atmel MEGA16U2 or the CH340G interface chip. For USB. There are known issues with fake FTDI chips.