A valid gas certificate (cp12) which is required by law and carried out on an annual basis.
A energy performance certificate (epc) which is required by law and is valid for 10 years.
An electrical certificate (NIC) is a landlords safety net to ensure electrics are of good working order however this is not a legal requirement.
If you are a freeholder of a property with communal areas, by law an asbestos report is required. E.g. A block of flats or HMO property.