No maker can support MKV because.. Are you ready for this? MKV files are containers. That is, you can't know what the video is encoded inside that MKV containers.
So my answer is no. Not all files should play. Often this is the first encounter for some with encoding and all about this is more than fits in this reply box here.
As to what to buy, the usual is your PC. It's very capable of rendering such things.
I want to buy a Panasonic Viera TV. There's the ST, the GT, then the best one, VT. They all have USB ports from what I understand. I have a few external hard drives full of bluray movies/tv shows. My question is, can any of those Viera TV's play MKV files from an external hard drive if it's plugged into the USB port on the TV?
If it can't. Is there a way to do this by buying something that you can allow you to do this?
I'd really appreciate the help.