Who has the most power and privileges in Linux?

Answer 1

That would be "root." THis user can do virtually anything on a Linux system.

Please note, however, that having "maximum permissions" is not actually a good thing. "Ultimate power" may work for comic book villains, it's poor usage on computers. When logged on as root, there's nothing stopping you from annihilating everything on your machine by accident. Linux won't wait for you to be "pretty sure" before happily executing a command.

But by keeping your normal everyday user as a regular user, admittedly likely also in the sudoers file, a command like rm -rfv / can limit the damage to stuff you actually have access to. A lot of people say this is backwards and your personal data is more important... this is true on a personal level, but computer security in most cases is actually NOT about protecting a user's data from its user or intruders, but from keeping the entire system from being compromised through a single user. May seem shortsided from a desktop sense, but for PRODUCTION machines this is absolutely essential.

If you're responsible you'd have yours and all users' data backed up, so that if the unfortunate happens and a user nukes their own home directory you're able to recover it.

To effectively stop user homes from being compromised, it's usually better to mount their individual homes as encrypted file-stored filesystems. Truecrypt is very good for this. If a user isn't logged in, then their home should not be mounted and their home directory file will be very well-encrypted and (If you use hidden volumes.) hidden from intruders who might have your "outer" password.