Carrentals.com is hands down my favourite car rental site. It is usally the cheapest site and if it isn't number 1 at a given point in time, it usually isn't far behind. The flexibility is great... I often book and then check back for lower rates as travel dates approach. Never had a problem cancelling and rebooking another reservation. It has been years since I've used hotwire for a rental car... in the cities I travel to they typically come in a few bucks higher than carrentals, and with zero flexibility for changes or cancellations.
Never had a problem with the rental agency either when making a booking using carrentals, although I am intrigued by autoslash. I'm going to check it out.