Cotopaxi used to be the highest active volcano in the world but it lost that distinction decades ago (though if traveling in Ecuador locals still quote it as such).
Tupungato (Chile-Argentina) and Sabancaya (Peru) are both higher and more active than Cotopaxi.
Cotopaxi at 5897 meters had its latest eruption in 1904 and some very minor steam eruptions in 1975.
Sabancaya, at 5976 meters has had many eruptions between 1990 and 1998 and is set up to go off again at any time.
Tupungato, at 6550 meters (though the active crater is a few hundred meters lower) had its last major eruption in 1986, and several minor steam eruptions since then.
Ojos del Salado, at 6880 meters is often quoted as active because of the steam vents, but this is very debatable, as by most definitions it is not an active volcano. By any definitions however, Sabancaya and Tupungato are certainly active volcanoes, and higher than Cotopaxi.