The standard media type is video/mp4.
The standard mp4 container format is commonly used for both AAC audio, and H.264 video + AAC audio. These have different media types, audio/mp4 and video/mp4, however often you want different applications for audio and video and on some systems it is only possible to associate a default application with a file extension. Therefore it has become popular in some circles to use the extensions .m4a and .m4v for audio and video(+audio), respectively, in an mp4 container. However this does not affect the media type, which already distinguishes these using the audio or video prefix.
A twist, however, is that Apple started using their own media type, video/x-m4v, for videos from their store, which are in an mp4 container and use a .m4v extension. This is set to open the video in iTunes by default. Sometimes that is necessary because the video uses DRM, AC-3 Dolby Digital audio, or other capabilities that are not commonly supported in an mp4 container, but which are supported by iTunes for files with a .m4v extension. If you rely on such capabilities then you may want to use this media type instead of the standard one.
Media types with no x- are standardized in an RFC and tracked by IANA. No media type with the name video/m4v has been standardized. Non-standard media types have a x- prefix.