VLC Media Player takes only 3 seconds to start and then you only have to select the file and VLC will play it.
It supports MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ... and DVDs, VCDs, and several stream protocols.
It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
In addition, it also plays incomplete files, which is really interesting if you are downloading a movie and you want to check if it is the real one or a fake.
All these features have made VLC Media Player (Formerly known as VideoLan Client) one of the most used media players.