Green Flag used to be very good. Haven't used them for a long time though, as I now have a car on Motability and it comes with RAC cover through Direct Line.
If you take your car abroad I found the AA to be most competitive. I was with Green Flag but they wouldn't cover a car over 10 years old outside the UK, and my Astra was 11 years old when I went for a blast to Russia and back.