I'll get in before the pedants, just to save you some time! HPI is a brand name, so the best 'HPI Check' would be from HPI the company.
To be honest, they all do a similar job, so I wouldn't worry too much which one you go with. That said, it's a small price to pay when looking at an expensive car, and I have always used HPI for my checks.
I believe for them to be truly of value, you have to enter the VIN as well; though I didn't personally do that for my recent purchase.