MKV is pretty much the standard out there today.
Most of the trackers with internal groups will be focused on this format.

If you are looking for 720p and up, you should focus on HD dedicated ones, like mentioned by Tassadar.
SD is harder to find, since most of the trackers that are movies specialized are more difficult to join.
I’d recommend HD-Torrents for 720p up, and X264.me for SD (if they ever come back up).