Always used The Auto Group and found them pretty decent.
They like dealers that prep their cars well and dont just "buy, clean and sell" they like to know you have either given it the "once over" or you have given it to a garage to do before you put it up for sale.
Compared to many Warranty companies that are not worth toilet roll, I have found that they do out shine many of the competition.
I also know that AutoGuard are pretty good too.
Hope this helps