Slipknot is indeed fucking garbage. They're the ICP of metal.
As for me, I don't really need to go to the gym since I work in a warehouse. We do have music playing all the time back there, but it's not always my playlists. Sometimes one of the other warehouse guys puts on their own stuff. One dude only listens to weird ass stupid rockabilly bands where they play the same shit the whole song while some chick goes "yeah" every few seconds. Another guy only plays punk and alternative rock from the 90s. So much fucking Green Day.
I usually just stick to a broad genre and randomize the playlist since I'm back there most of the day. I have a power/folk/progressive metal playlist (Stratovarius, Sabaton, Rhapsody of Fire, Kamelot, Hammerfall, Evergrey, etc), a female-fronted metal playlist (Within Temptation, Nightwish, Amberian Dawn, Delain, Kobra and the Lotus, Epica, Xandria, Katra, Tarja, In This Moment, Sister Sin, etc), a classic metal playlist (Sabbath, Dio, Ozzy, Metallica, Maiden, Priest, Thin Lizzy, Deep Purple, BOC, etc), a classic rock playlist (Dire Straits, Zeppelin, Pink Floyd, The Doors, Eagles, Beatles, Rolling Stones, Bob Seger, etc), and a few others.
There are a few rap acts I like (Ice Cube and NWA mostly) and I will occasionally listen to some John Denver or Charlie Daniels. After all, I am pretty redneck. But mostly it's various metal subgenres and all the music my dad got me into as a kid.