Updates from an earlier comment:
It is also known as the world’s third highest active volcano after volcan Ojos Del Salado (6,893 m / 22,615 ft.) and Tupungato (6,550 m / 21,490 ft.) in the Argentine/Chilean border.
There are some other active volcanoes higher than Cotopaxi. Sabancaya (Peru), for example, is very active and was last surveyed at 5976 meters, but with all the recent eruptions, it is thought to be over 6000 meters. Either way, it is higher than Cotopaxi.
Guallatiri is at 6063 meters and is very active as well. It may be the highest volcano that has actually be seen to erupt lava in recent times.
It is debatable whether or not Ojos del Salado is an active volcano. It is a volcano for sure and has some fumarole activity and steam vents. Some consider it to be active, while others do not.
Part of Tupugato massif is active, but it's not the main summit, but a subsidiary summit known as Tupungatito which is active. Tupungatito is around 5900 meters (some sources say 6000 meters).