I personally find Momondo to be the best. It is an aggregator, though, and doesn't sell tickets itself (unlike Priceline, Expedia, and Orbitz). Dan refers to Priceline because he gets a referral fee for doing so. It helps cover the cost of providing the free service called dansdeals.com
I will note that Momondo often pulls results from sites with less generous cancellation policies and customer support than Priceline, Expedia, and Orbitz, so caveat emptor.
In terms of ownership, theres been massive consolidation in the industry the past few years, and its basically down to 2 parent companies. The Priceline Group owns and operates several travel fare aggregators and travel fare metasearch engines including Booking.com, Priceline.com, Agoda.com, Kayak.com, Cheapflights, Rentalcars.com, Momondo, and OpenTable. Expedia, Inc. owns and operates several international global online travel brands, primarily travel fare aggregator websites and travel metasearch engines including Expedia.com, Hotels.com, Hotwire.com, trivago, Venere.com, Travelocity, Orbitz, and HomeAway.
Lastly, proper grammar would facilitate more effective communication.