@paok1970,
Here is one "easy" way to decide whether it should be "who" or "whom": Rearrange the words in your mind.
1. "Who/whom do you live with?" In your mind, think: "You do live with who/whom?" You know that "with" is a preposition, so you need the objective form ""whoM" after a preposition.
2. In your mind, think: "You do like who/whom best?" You know that the objective form follows a verb such as "like." Therefore: "WhoM do you like best?"
*****
If you wish to speak traditionally "correct" English, you may wish to use "whom" whenever it is called for.
*****
CAREFUL! Here is a sentence that will confuse even people who usually speak "correct" English.
"Give this book to whoever/whoMever answers the door."
Some people will say "whoMever," for they think: "I know that 'to' is a preposition, so I need 'whoM.' "
In fact, we need "whoever," for it is the subject of the verb "answers."