User blog comment:Deshiba/Solo Queue: Skewed perceptions./@comment-5557556-20130730173729

I quote our wiki.

"Champions with extremely high defense, health, and crowd control abilities, but low attack damage or poor ability power scaling. Tanks are usually champions that combine high amounts of health, armor, and magic resistance. If left alone, tanks are able to absorb high amounts of damage or create disables on their foes, which make them great harassers as well. In battle, this type of champion is usually expected to start/initiate (or at least be in the front of) the team fight; soaking up damage, and helping focus certain enemies (carry and mage champions). Due to their low damage output, they must rely on their assassin, carry, fighter, and mage champions to finish the enemy off or chase them down when they initially weakened them. As a result, tanks are not expected to gain high kills but more assists. Tank champions should coordinate team fights by telling team to retreat or advance when necessary since they lead the charge. They are also meant to be a protector of the team, and must be willing to sacrifice himself/herself for vulnerable allies if necessary."

So that drops out Blitzcrank, Cho Gath, Gragas, Hecarim, Jarvan, Jax, Lee Sin, Malphite, Nunu, Rumble, Singed, Skarner, Wukong and Yorick.

The only champion that I would demote from tank to fighter is Volibear since 2 of his spells are actually only dealing damage and are not disruptive in any way nor they give him some tanky capability.

Closest to promoting a champion into a tank is Blitzcrank but he is too squishy to be one. Even if you build him like a tank.