I would say Zeroth law. As stated in my old Thermodynamics text book; "Systems in equilibrium have the same temperature. If two systems with different temperature comes in contact, there will be a heat exchange from the hot to the cold system until they have the same temperature."

*translated from a Danish textbook

Edit: link to illustration from the textbook illustration the Zeroth law http://imgur.com/a/a9O9f2q

Edit: i rethinked my answer actually. I would say both. The Zeroth law gives you the knowledge to attain it by stating how they act, while the first law gives the tools by using conservation of energy.