Well, truth is there isn't one.
The best one will be the one that the provider of the product or service you are about to purchase with....Whoever it is that they use to perform credit checks.
Lets work through an example.
Lets say you will be changing your car in the next six months. You always purchase using higher purchase(HP), which means the finance company will 99.9% run a check on you. The finance company will use a Credit Reference Agency to understand how you manage your finances.
Now, the thing here is they will only use one provider. The trick now is to make sure when you go talk finance with the retailer you would want to know which agency the finance company uses. It then also helps to know which credit reference you have a better score with. It is then a case of selecting the right retailer that works with the agency you have best score with to maximise your chances of success.
The table below show the main providers of products and services and which credit reference agencies they use.
Agency Used to check your Credit Score
HSBC (inc. First Direct)
Rate Setter (Peer to Peer)
Royal Bank of Scotland
Note: The data for this table came from a MoneySavingExpert article published in March 2016. For more accurate information always check with your lender directly.