Windows

 ·  VeraCrypt
 ·  BitLocker

I recommend VeraCrypt over BitLocker. There is no definitive proof that BitLocker is backdoored but without the source code we just don’t know. Plus it saves a copy of your key to Microsoft’s servers by default which is a very insecure practice

Linux

 ·  LUKS

Most distros will have an option to enable LUKS encryption during installation so do it then

Mobiles/Tablets

All modern Android and iOS devices come with file-based encryption enabled by default now. You just need to choose a good passcode when setting up the device. 4 digit and 6 digit PIN’s can be brute forced, use a 15 character password instead including both letters and numbers