There's not a simple answer to this question. Different people have different tastes, so each person might like a different pen. Until now, there's no unanimous agreement on which pen is the best for spinning. I personally prefer the Pentel RSVP, but there are others who said the Pentel Sharplet-2 is also pretty good, and even people who like just a simple No.2 pencil. There are several aspects to consider when choosing a pen: the length, the thickness, the center of balance (COP), and the texture.

Most people will agree that a longer pen is better for most tricks. The thickness, however, is pretty much a question of taste. Some people like thicker pen while others like thin pens. I'm still doing some research on this, but I'm almost certain that some tricks are better done with a thick pen, while others require a thinner pen. Now, the COP should be preferably towards the center. This allows you to easily perform both normal and reverse variations of a trick. The texture is another factor that is very controversial. Some people prefer a pen that has a better grip while others prefer a more smooth pen (meaning more slippery). I believe this is also relative to the trick you are doing.

If you are trying to choose a pen, a good rule of thumb is to go with one that is more balanced. That means that the pen should not be too long or too short. It should not be too thick or too thin. It should have a COP towards the center, and it should not be too grippy or too slippery.