There are 3 main styles when you watch a movie in VR, Full 360 video, 3D video, and regular movie format. When watching regular movies you really need something surrounding the video or you are just floating in space with a screen in front of you. This can get really disconcerting so most apps try to create a space for you to enjoy.
I haven't mentioned video quality here as almost all the apps require an internet connection to stream and will dynamically alter the resolution as they go. However, due to the nature of VR, even low-resolution videos look good as they are so close to your face.
Here are some of the best apps to watch standard movies on.