I've been with the RAC for years and have always been happy with them, but I've never used anybody else so I don't have anything to compare them to.
I guess it partly depends on your priorities though. The reason why I chose RAC in the first place was a comparison which said they did the highest percentage of roadside repairs, as opposed to just taking you home broken. Green Flag are supposed to have the fastest average response time, which might be important if you often travel alone. If you have more than one vehicle you'll want personal as opposed to vehicle cover.