There is no single one. The insurance companies offering the best bargain prices vary from one motorist to another. Some are specialists in providing cover for experienced motorists with good claims records and few if any motoring convictions; others prefer to cover high-risk drivers such as teenagers, learners, and convicted drivers. Others concentrate on classic, high-value or prestige cars; and there are those that try to attract female motorists only by offering benefits such as handbag cover or discounts for approved baby seats.
Since we all have different risk profiles no single insurance company can ever offer the cheapest prices to everybody, which is why to find the best bargains a price comparison service, which compares the cheapest quotes from multiple companies, can really save you a lot of money.