Artificial horizon [for pitch and roll information.]
Airspeed indicator [to keep from diving/and or slowing to a stall situation]
Turn and bank indicator [for making turns and the use of proper rudder input
to keep the aircraft from skidding]
radar
Airplane pilots "walk on clouds".
When a pilot has to fly through thick clouds, many instruments come into play. These instruments help the pilot navigate the clouds and are known as GPS and Distance Measuring Equipment which prevents collisions in mid air during these weather conditions.
Pilots use instruments such as altimeters, airspeed indicators, and attitude indicators to measure altitude, airspeed, and aircraft orientation. These instruments help pilots navigate safely through airspace and maintain control of the aircraft during flight.
The best answer is "all of them." But in reality, the ones scanned most frequently would be the artificial horizon, the airspeed indicator and the altimeter. The vertical speed indicator and the compass are also critical with the VSI giving you the earliest indication of a climb or descent, and the compass confirming level wings and/or no slipping. These instruments are used to maintain primary control of the aircraft, but additional instruments are required for navigation to a specific point.
It might shake only when passing through turbulence. Clouds may or may not be turbulent. Pilots learn which are and which are not turbulent. Clouds that occur in even layers are very smooth. Thunderheads and lenticulars are often violent and are usually best avoided.
Pilots use mostly electronic equipment. If electronics go out they use instruments.
GPS
A flight passing through overcast cirrus clouds may encounter light precipitation such as drizzle or very light rain. Cirrus clouds are high-altitude clouds made up of ice crystals, so the precipitation they produce is typically very light. Pilots should be prepared for the possibility of minimal precipitation when flying through these clouds.
Cumulonimbus clouds can pose hazards to aircraft in the form of severe turbulence, strong winds, lightning strikes, and hail. These clouds are associated with thunderstorms and can lead to dangerous flying conditions, making it important for pilots to avoid flying through or near them.
there isn't any
Thick clouds are really just a light mist, flying through them is similar to driving through thick fog, it doesn't affect the performance of the aircraft in the least. It does however affect the pilots ability to see, and most aircraft are equipped with instrumentation which allows the pilot to maintain control and positional awareness even with all windows blotted out. Aircraft which are not so equipped are forbidden from flying into clouds. Aircraft flying into and through clouds can only do so under more restrictive rules requiring constant contact with air-traffic-control.