One very cool topic when it comes to tanks is armor. Those went from, essentially, thick sheets of metal to being made out a combination of materials that are regarded as military secrets (the exact "recipe" is the military secret, but the materials are somewhat common).
Armor has become so efficient and improved that you can actually have great armor that is considerably lighter than it was for tanks during the height of World War II. Throw in more powerful engines and you've got lighter, faster and stronger tanks with more powerful guns thrown in for good measure.
The tank has remained viable on the battlefield simply because the design has been refined, improved and updated over the years. There are few weapons types you can say that about.