I like BizTravel. It is by far the easiest out of any other site out there. It saves all of your preferences (FF #, car type, seat type, airline, car #, etc.)

It is a one-stop page to get a complete priced itinerary including air, car, and hotel. You can also book everything on the page with 2 clicks. It also saves your discount numbers and automatically quotes a rate with the discount.

PLUS, it automatically requests upgrades based on your status!!

It also checks the fare every day to see if it has gone down. If so, they tell you how to get a refund.