Here’s what I’ve settled on for air travel.
Start with Expedia or Orbitz which work well and have all the options. If your search isn’t too complex, you might try Qixo before you buy. Its UI sucks, is much less flexible, and is incredibly slow…but it sometimes finds flights Expedia doesn’t. Travelocity’s UI is frustrating and they use popunders – don’t bother with it.
If I find something, I sometimes go right to the airline and buy the ticket from them, not to save $5, but to avoid the situation where the travel site and airline pass the buck to each other in case of changes to be made.
Search the “discount” airlines like Southwest separately since they aren’t included in any of the travel sites. Nowadays the major airlines are worse experiences than these “no-frills” ones, if they have your route. I use Southwest when I can because I don’t have to worry about fees for changing or not using tickets, kind of like the way the world should be.
In terms of comfort in economy class, Southwest, AA and United (only if you pay for Economy Plus) are a little better. Continental and Northwest are a little worse.