The last few years, I have been flying only Southwest, and they are not on any of the booking sites, so I book directly with the Southwest web site.
In the past, I used to book air with USAir and Metrojet through Travelocity and had good experiences, but haven't done it in years.
For car rental, I always book with the rental company's own web site, using codes and coupons from various places. I have gotten the best deals the last few years from Alamo and Thrifty, by using coupons and codes from
Sams Club membership warehouse (Costco and BJs have similar discounts). Other great discount codes can be found on
www.MouseSavers.com
Tips for car rental: Rental companies don't charge deposits, so you can book, cancel, and rebook as many times as you want if a better rate comes along. The best way to get the lowest possible rate is to keep checking back with all of the major companies at least once a week. Remember to look at the total after taxes and fees, not just the weekly rate, since this can often be misleading. And never cancel one until AFTER you book the lower one, just in case something glitches.