I had Greenflag; they let me down massively once and I was waiting around from 1pm until 7pm. Decided to renew with RAC as they were offering their full cover for £69. Haven't used them yet but for me all these companies are the same. 99% of the time there will be no issues, but it's that one time where they end up letting you down and you'll lose all faith in them!
To be fair to Greenflag, I called them out 3 times in a year and only once they let me down. the other 2 times they were really good, and they did refund my cover (and allow it to continue, so essentially free of charge) because of their screw up that had me stranded for ages.