LoveVery personal. Used between lovers, family and close friends.
CheersVery casual. Used between friends. Can be used between coworkers if you want to appear very casual, at the risk of sounding unprofessional.
Take careCasual and used between friends.
Thanks or Thank You'Thanks' is very casual and 'Thank You' is formal. Usually used in a business setting. 'Thank You' is a great way to end an email to a stranger, especially if you are making a request or asking a question.
Yours or Yours Truly or TrulyCasual or Business casual. A more personal, but still polite way to end the letter. You've probably met the recipient in real life.
Best or Best WishesCasual or Business casual. A polite way to end the letter. Can be used between friends or strangers.
Regards or Best RegardsBusiness casual or formal. You would use this if the recipient is someone you haven't met in real life.
CordiallyFormal. Used in business correspondence when the topic of the letter is serious. Sometimes used if there is a conflict between the sender and the recipient.
Using an ending that is too casual for the situation may not always be inappropriate. Conversely, using a casual ending can show your self-confidence. But to do this requires you to gauge whether the recipient would like a more casual conversation.