FAR 91.205(b) lists the required instruments for day VFR and are required to be in working order.

Day VFR
T - Tachometer
O - Oil Pressure Guage
M - Manifold Pressure Guage for Altitude Engines
A - Airspeed Indicator
T - Temperature Guage for Liquid Cooled Engines
O - Oil Temperature Guage for Air Cooled
F - Fuel Guage for each fuel tank
L - Landing Gear Position Lights for retractable aircraft
A - Altimeter (Pressure)
M - Magnetic Direction Finder (Compass)
E - ELT (refer to 91.207)
S - Seat Belts and Shoulder Harnesses (depends on manufacture date)

Night VFR
F - Fuses (Set of three for each type installed in airplane)
L - Landing Light (if for hire)
A - Anticollision Lights
P - Position Lights
S - Source of Power (Battery, Alternator/Generator)

The ones in bold answer are in the six pack instruments. Amazingly there are only two that are required either for day VFR or night VFR. This may not be the complete answer. Depending on how your aircraft was certificated other instruments may be required. That information is located in the aircraft equipment list. Any instrument with a "-r" is required.