These are the ones I personally use and recommend any Linux user:
gtkpod - For anyone that still uses iPods or has any old ones and want to stick music in it without using anything iTunes related.
Handbreak - For ripping dvds. Also is a handy video converter (any format to MP4/MKV)
Audacity - For audio editing
LMMS - For music making
SimpleScreenRecorder - If you wanna record your entire desktop screen or if you play video games and want to record your gameplays, this is the best lightweight recorder around.
jstest-gtk - Easy way to calibrate your joystick/gamepad for video games.
KeePassX - Handy password manager.
Furius ISO Mount - As the name implies, you can mount ISO files as if you had an actual CD/DVD in your computer.
ShotCut - Handy Video Editor. Comes in an AppImage.
balenaEtcher - Create bootable OS USB drives
Transmission - Simple Torent client.
Sound Juicer - Rip audio CDs
QJoyPad - For mapping keyboard buttons to gamepads/joysticks if you're playing a PC game that lacks that kind of configuration.