Airline before you leave baggage claim (IME I've had great sucess - including BA couriering me a brand new suitcase within 72 hours, same brand but actually a much nicer model than I had to begin with, plus mine was around 8 years old - and absolutely dreadful responses, like the company who gave me the forms, filled them in with me in the baggage hall etc etc. and then 3 weeks later wrote me a letter saying 'you filled in the wrong forms... oh and btw, the 7 day claim window is up, so tough luck' - fortunately my travel insurance fully covered me.