What state do you live in, that is probably a good starting place. NY based policies are some of the best when it comes to rental cars, while other states, not so much.
Many credit cards cover rental cars, but most are secondary, meaning filing through insurance has to occur first, followed by the card picking anything up that insurance doesn't (ie. deductable, excess above and beyond coverage). Many credit cards don't cover SUVs, larger vans (minivans are usually covered) and higher end cars (ie. BMW, MB, Volvo, Lexus, Jaguar, Cadillac SUVs, Lincoln SUVs, etc.)
AMEX offers a Premium Rental Insurance coverage, for a fixed rate, regardless of the rental (as long as the rental is charged to the card). It is primary coverage and covers most vehicles (including those excluded in the second paragraph above).