By plane
Start with the worst first. Cambodia goes through national airlines like there's no tomorrow. Carriers that have faded from the scene include Angkor Airways, First Cambodia Airlines, Kampuchea Airlines, Mekong Airlines, President Airlines, Royal Air Cambodge, Royal Khmer Airlines, Royal Phnom Penh Airways, and, most recently, Siem Reap Airways -- and that list is for just the last ten years or so.
As it stands, there is no Cambodian airline flying in-country, but Thai carrier Bangkok Airways flies the very popular Phnom Penh to Siem Reap route.