Fast-stabilising skin temperature sensor
I would consider a thermistor or platinum RTD in contact with either the skin or something very thin between the skin and the sensor, the whole thing covered by an insulator (there are some very high performance foam-like insulators) and an adhesive patch over the lot. The wires to the sensor must be thin. Optimum accuracy might be from using something like AWG 36 wires (2 or 4) that are of low thermal conductivity, such as manganin or phosphor bronze, but using copper and "heat sinking" enough length to the skin is probably enough. 

Key points- use a high performance sensor such as a Pt1000 thin film type, minimize heat loss out the leads, and maximize thermal insulation overtop of the sensor by using high-tech insulation materials.