I do not really have much to say about car rental companies other than to find one and stick with them. Most are identical in amenities and service. All car rental companies offer some kind of rewards program and whatever car rental company you choose you should ensure you sign up for the rewards program. Many companies have preferred car rental companies and some will even pay for you to join a rewards program (you read that right, car rental reward programs cost money). Be sure to check with your company to see what options are available to one.
Though most care rental companies are available at any destination you can think of, it is still a good idea to see what companies are available at your most common destinations when choosing a company.
The one comment I would make is that as reward status increases that you be sure to read over the amenities offered to you and take advantage of them whenever you can.