If you are serious about video playback, and need to tweak and customize how video is played then Zoom Player is definitely for you. Sure, VLC and MPC play "everything" and are free. But honestly - their user interface is a mess, it's hard to figure out where to customize playback and settings. They work fine for a basic user who just wants to play a file, but lacks a lot in the options and features.
Zoom Player has a ton of features, while making it easy to find the basic settings. The Advanced menu is huge, and it also has a search function to easily find the settings you want which is a huge bonus.
The intergrated Install Center allows you to download all the codecs you need to play 99% of all video out there: LAV Filter, FFDShow, MadVR, AC3filter and so on. There's no need to download those huge and confusing "codec-packs" that in my experience just mess up your playback.
There's a free version as well, available on the developer site, and it does basically everything the Steam version does, but you get some extra features here, and you are supporting a quality product (which IMO is important as well).
The most lacking feature might be the network streaming, like VLC has. But then again VLC was built from the ground up to be a networked media player.
I've been using Zoom for years now, and it outclasses all the other media players I've tried.
Personally I'd rather get it from the developers themselves, but having it in Steam is a nice addition.