The T-54/55 tanks were a series of soviet post-WWII tanks, and, while some sources classify it as an MBT, the soviet army's official classification for it was that of a medium tank. They have been extensively upgraded since then and are still in service with some armies around the world. Given its popularity and ease to porduce, it eventually became the most produced tank in history, with production numbers ranging from 86,000 to 100,000.
They were armed with an 100mm rifled gun, had a 581 HP engine that gave it a power/weight ratio of 14.6, and propelled it to up to 48 km/hour.
It is robust and easy to operate and produce, yet having a very good performance for its time, a factor partly accomplished by ignoring the crew's comfort. It used mainly sloped armor in the front, and so didnt have to have much armor thicknes, achieving low weight. Its D-10 gun could penetrate up to 160mm of armor with APHE rounds.
It participated in many wars of the 20th and early 21st centuries, although not achieving a good performance in some, mainky due to it having to fight against other tanks decades ahead of it.