RENTALCARS.COM
We almost always search up our car hire on rentalcars.com. They are really good, and unlike many other rental car search engines, their estimate prices are actually really close to the actual price you will end up paying. Oh you thought that the prices are realistic? Not always.
Failing that, these are the companies we more often than not end up booking with: Hertz (in the EU, AU and US), Enterprise (in the UK and US).