Kayak and google flights are the only ones that provide a radically different experience from the others, which I find are all the same in terms of routing and pricing options and method of search (expedia, travelocity, bookings, flightcenter, redflag, lastminutetravel, itravel200, a couple others). To me that list is all extremely similar in terms of a travel surfing experience. I normally do my preliminary research on kayak and google flights, then triple check with expedia before making a decision. I've booked through expedia, redflag and travelocity, at various times, as well as kayak and even handatravel. No one site seems to consistently outshine the others in terms of deals.
I'd say, pick whichever site you like the look and feel of, and cross-check your trip on one other site. Beyond that is probably a waste of your time.
Sadly, after several weeks of researching our next major trip, I found the best deals and the ONLY direct flight for the destinations we were looking for, at......... I hate to say it........ aircanada.com.