As others have said, The FireTV2 is a very nice box. Easy to use, great compatibility, and lots and lots of support. Price won't break the bank, and it has the hardware to last quite a while.
If you want complete customizability, there are piles and piles of cheap Android boxes available. You can get a nice box with an Amlogic S812 quad core chip(best available right now in Android boxes), Android 5.1, 8-16GB storage, 2GB RAM, Gigabit ethernet, dual channel wifi, HDMI, 4K capable, optical out, SD card, remote, etc. Anywhere from $50-100. But most come preconfigured with bloatware and junk you'll never use, so if you go that route prepare to erase the device and start from scratch, and it's likely going to take a great deal of tinkering to configure and maintain. Here's an example:
http://www.amazon.com/Zenoplige-Amlo.../dp/B015CRM4JA
You can search the Related Items on that Amazon page to view a bunch more. Many of those will have slightly better hardware/connectivity than FireTV2. But like I said they will require more hands on configuration.
If you want to step up from that and plan to do more than just movies/TV, I would recommend the NVidia Shield. It's $150, but allows for gaming and more.
A step up from that would be an Intel NUC. A PC in a tiny box. But that's getting up ~$400-700. Best possible performance though.