No such thing as best shop, they all have their good and bad points. The same applies to a computer,and your specification or needs would have the deciding factor.
Straight off the shelf: PC World/Curry's/Comet and similar superstores. Built to order via mail-order, internet,telephone sales: Novatech, Mesh,Evesham,Dell are but just a few of the many suppliers.
Provide your requirements, and someone may recommend a good supplier and product.