It really depends on what behavior you want from your bullets.
The most common node to use for bullets is Area2D. Since you only need to detect when the bullet hits something and then it's gone/explodes/etc. there's no need for collision.
If you want your bullets to ricochet, then you might want to make them rigid or kinematic. If you want direct control over their movement, kinematic is a better choice. If you want the physics engine to control them, use rigid - but you'll have a lot less fine control of their movement.