I mostly use Roku and Samsung Smart TV. Almost all connected/smart devices have Netflix and YouTube, but those two also have Amazon Streaming. Sometimes I'll use Chromecast, but mostly just for casting from my phone, and sometimes for YouTube if I've browsed to a video from my phone. On occasion I use Apple TV because HB0 N0W is on 5.1 surround, whereas on Roku it's only in stereo (although soon it will be in surround sound too).
If I were recommending a device, it would be Roku. A lot of the apps are not as fancy as, say XBox or PlayStation apps (although that's going to change soon), but it's reliable, affordable, and has the widest selection. I wouldn't buy a stick though -- the WiFi is flaky. Get a Roku 2 or 3 2015 version (
or pre-order a 4).