For rubber caps, take the steel band off cap with a flat head or common screwdriver. For PVC caps, unscrew with a pipe wrench, channel lock, or large crescent wrench.
A brass cap is the same as the PVC caps. Most brass cap threads will lock into cast iron pipe making it difficult to remove if not impossible unless threads were lubricated prior to being installed. Sometimes the best and easiest way to remove the brass cap is to bend it slightly enough to drive a flat head screwdriver into the threads and pop the cap off the clean out. "Do not drive cap into clean-out or drop it down into the cleanout as this will most definitely cause a stoppage.