I would love to get a recommendation for a head for sirui p-204s monopod.
There are three basic choices regarding heads used with true monopods. Which one works best for you will depend on what you want to place on the monopod and what you want to shoot with it as well as personal preference.
Many monopod users use no head at all. With larger, heavier lenses that will most often be used with the lens' optical axis pointed horizontally this is a good choice because attaching a very heavy lens directly to the monopod eliminates the instability that adding an adjustable head can introduce. This is particularly the case since most monopod heads are rated for fairly low load capacity. A lens such as the Canon or Nikon 600mm f/4 lenses will weigh more than most tilt or ball heads are rated to support.
The nature of a monopod allows panning right to left or left to right simply by rotating the single leg.
The lens and camera can be rotated within the tripod collar's ring to transition from landscape to portrait orientation. This is often the case because one of the chief reasons for using a monopod is to help support the weight of a heavy lens.
Monopod tilt heads only rotate on one axis. This can be used one of two ways, but not at the same time. The tilt can be used to raise and lower the point of aim of the lens or it can be turned 90° with respect to the optical axis of the camera and used to allow switching from landscape to portrait orientation.
The former is usually the case with larger lenses that have a tripod collar that allows the lens and the camera attached to it to be rotated to transition from landscape to portrait orientation.
When using smaller lenses without tripod collars the camera is attached to the head which can be used for either tilting the lens up or down or for tilting the camera to the side for portrait orientation depending on which way the camera is attached to the head. Either way, to pan from right to left the entire monopod can be rotated rotated on its own axis.
Ball heads are difficult to use with heavier lenses for two directly related reasons. The first is that the weight of such lenses make them want to flop around when the tension on the head is loosened for adjusting the position. The second is that, unlike a tripod, a monopod must be help up to keep it from falling over. These two things conspire against the user. If one hand is holding the monopod to keep it from falling over and the other hand is operating the adjusting knob on the ball head, which hand is holding the camera and lens to reposition them and keep them from falling over?
This isn't as much an issue with lighter cameras/lenses that are typically attached via the camera rather than the lens. The hand supporting the camera during adjustment can also hold the entire assembly up enough to let the leg dangle in mid-air as the other hand operates the levers/knobs on the ball head. Lighter weight monopods meant for those lightweight cameras are sometimes sold with ball heads.
A pistol grip head is a specialized type of ball head with a different way of releasing/tightening the tension on the ball. A few folks like to use them with monopods, but I rarely see anyone that does. To get it to work with heavier lenses a fairly high end pistol grip must be used. I often use a pistol grip with a tripod, but not with a monopod.
There are some 'monopods' with 'feet' attached to the base to allow them to stay upright, at least for a short interval, on their own. These aren't true 'monopods' in the sense of a single legged camera support. Often these types of 'monopods' are used with either video heads or three-axis heads because the feet prevent panning by rotating the main shaft. These are usually used with smaller lenses without tripod collars.
I have occasionally seen online references to using a small gimbal head with a monopod, but such a use is fairly rare and I've never actually seen it in person.