For me it depends on my destination. If I am traveling within China, I always use Ctrip
, as they generally show more flight options than Expedia or Travelocity (e.g. PVG
has one flight on Expedia and Travelocity vs 10 - 15 flights on Ctrip). Plus Ctrip generally include non-major airlines in China such as Juneyao, Hainan, China Spring, etc. Ctrip also lists all the different fare categories for the flights, e.g. you can choose to fly cheap and not earn miles, or you can pay a bit more and earn miles.
For international flights I first go to ITA
and check out what the fares are like and I compare to Expedia and Travelocity, then I will generally just go directly to the airline's website and book there. However, there is an exception to this rule. If I am booking with a Chinese airline I generally prefer to book with Expedia and if my travel plans are not set in stone (e.g. I might need to change the date or destination), then I always book on Expedia as they have a 24 hour grace period by which you can cancel your flight without incurring any fees, and I've done this at least 4 or 5 times during 2010.
However, one thing I hate is that on Expedia or Travelocity you cannot choose the fare bucket, but instead they will just offer the cheaper fare. So if at all possible, I will check which fares earn miles and then just do it the old fashioned way by walking down to the airline ticket sales office (most airlines have ticket sales offices in Asia, but I can't remember when was the last time I saw one of these in the US).