What's best depends on your vehicle and your usage.

I've had a Coleman stainless 54-quart cooler in my Honda Element the past two years. It serves as a trunk and grocery hauler in addition to occasionally being packed with ice.

It may not be as versatile as a soft cooler but it is far more sturdy, better insulated and holds more than any soft cooler I've seen.

Something I have found very useful - especially since DC instituted a grocery bag nickel tax a couple years ago - is this Mountainsmith Cooler Cube. I have a couple and use them for grocery bags and for ice haulers when we camp. The cooler cube folds down pretty flat.

http://www.altrec.com/mountainsmith/cooler-cube?sku_id=521062&cm_mmc=Mercent-_-Google+PLA-_-Mountainsmith-_-51921-521062&mr:trackingCode=E6FAE886-B115-E011-B31E-001B2163195C&mr:referralID=NA&origin=pla&mr:adType=pla&gclid=CPTcv_qKlK0CFUTc4Aodvz0FlQ