Which are the most common quick release plate systems?

The camera has a hole in the bottom that will be meant to take either a 1/4-20 UNC or 3/8-16 UNC threaded screw.

Most attachments for this will either come with both types of screws or will necessitate the use of an adapter if what the item comes with is not fit to your camera. This monopod, for example, has a reversible screw for both. Point is, these attachments are standardized so the tripod world is your oyster, so to speak.

The quick release plate will attach to your camera via one of these screws - so you can use the same plate across any of your cameras. Or, if you're lazy like me, you'll buy extra plates and just keep 'em on your bodies.

The plate will be designed to fit whatever head you're using. They could be custom designed for the head or could be something more standard, like the Arca-Swiss style plate.

That being said, I've never tried to mix brands and I have heard stories of one brand's arca-swiss plate not quite meshing well with another's arca-swiss head, even though those should be universal.

To summarize - because of the universality of the attachment screw, don't let this impact your tripod head decision. Buy that for the features you want and then worry about the attachment, whether you need an adapter or not.

If you want universality in the QR plates, then go for a head that supports Arca-Swiss style plates. Though again, be warned that that is no guarantee of a great meshing between the head and plate if you choose to mix brands.

Other than Arca-Swiss style plates, which are not standardized and can come in wide variety of sizes from various makers, the most common type of QD plate with standardized dimensions is probably the Manfrotto RC2 plate and receivers.

Arca-Swiss plates come in several widths and lengths. The most common width is about 1.5 inches (38.1 mm). As long as the width is the same and the beveled angles on the edge of the plate are close enough to the angles of the groove on the receiver, plates from one maker and receivers from another maker may be used together. The exact angles of the beveled edges that run along the sides of the long dimension do vary from one brand/maker to the next. Slight differences between the angles of the plate and the angles of a receiver can usually be ignored, but there are cases where a plate from one maker and a receiver from another maker don't line up well enough to be used together. When considering using an Arca-Swiss style plate from one maker and an Arca-Swiss style receiver from another maker, one should always test the compatibility in advance of "mission critical" usage.

Many Manfrotto products are offered with RC2 connectors, but there are also a plethora of third party heads, plates, receivers, and knock-offs of other Manfrotto products that use the same connector.

Manfrotto now offers the 200PL-PRO plate that is compatible both with RC2 receivers and most 38mm wide Arca-Swiss clamps.