The Phantom is the standard for higher-end modern "stock" players. Unlike most people who play paintball, stock players use old-fashioned "pump" guns that must be re-cocked after each shot. They also typically limit themselves to 12-gram CO2 cartridges (like the original markers used) and a 10- to 15-shot magazine tube located on the top of the marker (also like the originals).
These markers are extremely customizable. They offer different barrel lengths, different shoulder stock options, different CO2 canister locations, different color schemes, and different handle styles. The average marker looks like one of the two pictured above, though. It is simple and useful. I would personally like a shoulder stock on it to help hold the marker steady for shots, because you will only be able to shoot one ball to the opponent's 15 and you have to make that ball count.
Here is a picture illustrating the number of upgrades available, and it only scratches the surface.