Best online air travel _search_ sites?

I have links bookmarked for pretty much all the major online air travel
booking sites I've ever encountered or heard of. That's the problem --
too many choices.

Would others want to list, let's say their top two or three choices of
online air travel sites (a) for domestic (U.S.) flights, and (b) for
international, with primary focus on ease of use, and comprehensiveness
and quality of information -- NOT on finding absolute lowest fares.