This will depend on what you mean by ‘cheapest’. With a PCP you’re only paying for the depreciation and not the car itself. This means that you’ll pay less overall than you would with a hire purchase (assuming that the car costs the same amount). However, you won’t end up with a car at the end, unless you pay the balloon payment which is likely to spike the cost right up.
With both PCPs and hire purchases, you’ll have to pay a deposit. This will typically be around 10% of the vehicle cost but if can opt to pay more you’ll reduce your monthly payments.
Personal car loans won’t require a deposit.
Interest rates (or APR), tend to be broadly similar across all products – around 4-7% is typical – and will depend in part on your credit score.