Well, there is no right or wrong way here.
It really depends on what you want to do. If the user of your app should only be able to watch the video and your app doesn't need to interact in any way during the playback or needs any information about whether or how the user watched the video, then an intent would probably suffice. And it's less work.
If you want to provide any additional features during the playback a VideoView would be better (and it's really not that much work to implement a simple videoview). Also with an intent you don't know what app the user will choose to open the video, so you have no guarantee how the user will experience your video...
If you implement your own video view you have full control, how it works and how it looks.
So if it's not the core functionality and you only want to show a video once or so, an intent is probably a good way. If it's an important part of your app and you might want to add features and if it would be a bad experience for the user if s/he chose a bad video player, I would definitely recommend doing the little extra work and impl your own video viewer activity...