A media streamer allows you to play content you already own and have stored it on a home network, USB drive connected to a computer, etc. A media streamer also helps you stream content from all the new streaming services including Amazon / Netflix (you can watch them with a media streamer too), HBO now...new streaming services are being introduced at a greater frequency nowdays.

The best media streaming devices will play content from as many sources as possible. So, just check. A good media streamer also provide search facility to search and prioritze content sources and content. They will also allow you to customize the interface and prioritize the services you use the most.

The ideal media streamer will not push a particular streaming service. It will give that power to you.

The best media streamer for normal use: Roku Streaming Stick (largest selection of streaming content, a clean and responsive user interface, and a useful search function.)

The second best media streamer for normal use: Roku 2 (same as Roku Streaming Stick, without Private Listening mode and downgrades to an IR remote, comes with ethernet port)

The best media straemers for Apple users: 32GB Apple TV (2015) -  supports most of the major streaming services plus voice search, not as flexible as Roku

The best media straemers for local media streaming: Nvidia Shield (great at playing media from USB drives, supports most file formats)