There are many video players available in google playstore but MX video player is easily the most popular and best video player.
I am using MX video player from last 2 years and is quite satisfied with it. The best thing I like in this video player is that you can watch any video in full screen on your smart phone.
You can also zoom in or zoom out the video by pinching or swiping across the mobile screen. There is a also a very good feature in MX player which is kids lock. Now your kids can also watch the videos and you dont have to worry about them opening another app or calling a person by mistake.
The another good thing about this video player is that it can support many video files such as avi, mpeg, 3gp, mp4, etc. However MX video player is best suited for HD videos in my opinion because it can play any HD video without any lag. So my experience with MX video player is very good.