One of the most popular video players on all platforms is the videolan player, better known as vlc. It truly plays almost everything that can be played, and also captures and streams video from various sources. It is not included in the default install of Slackware, but it is easy to install.
A longtime Slackware contributor, AlienBOB, maintains updated builds of vlc at http://slackware.org.uk/people/alien/restricted_slackbuilds/vlc. He offers a SlackBuild for it, but also downloadable pre-built Slackware packages (one for people who limit themselves to USA patent laws, one for the rest of us) that can be installed with
Even with a pre-made SlackBuild, building vlc is no small task, so it's a lot easier to just grab the vlc package and install it pre-built.
Realistically, vlc has no greater capability than mplayer, but to most people it is more familiar and has an easier interface.