Ultimately, the best pickup for the middle position of a 3-pickup guitar is the one that sounds best to you. You should experiment with different pickups until you find the one that gives you the sound you want.
Here is a more detailed description of each type of pickup:
Single-coil pickups: Single-coil pickups are the most common type of pickup used in electric guitars. They are typically made with a single coil of wire wrapped around a magnet. This design produces a bright and twangy sound that is perfect for country, blues, and rock music. However, single-coil pickups can also be susceptible to noise and interference.
Humbucker pickups: Humbucker pickups are a type of dual-coil pickup that was invented to reduce the noise and interference that can occur with single-coil pickups. Humbuckers are made with two coils of wire that are wound in opposite directions. This design cancels out the noise that is produced by each coil, resulting in a thicker and more powerful sound. Humbuckers are often used in heavier styles of music, such as rock, blues, and metal.
P-90 pickups: P-90 pickups are a type of pickup that is somewhere in between a single-coil and a humbucker. They are made with a single coil of wire that is wrapped around a larger magnet than what is used in single-coil pickups. This design produces a sound that is brighter and twangier than a humbucker, but it is also thicker and more powerful than a single-coil. P-90 pickups are often used in rock and blues music.