No Matter What Game, No Matter What Game Type, The Tankiest Characters...

...are always the ones that can heal themselves or constantly apply the equivalent of heals to themselves (extremely short cool down shield). Unless there is a character that can somehow be nearly permanently invincible without a workaround, self-healing or the equivalent of it is always the most sure way of being durable in a game. Why? Easy. Your effective health pool.

If your character does not heal themselves, they have a finite health pool. Once damage is taken, it sticks. Unless you retreat, it will stay on you and you have that much health left. If you can heal, you can immediately recover that health, effectively increasing your health pool for that fight by the amount you healed cause the only way to undo healing is through damage. That is why, going into the future, I would recommend that anyone looking for durable champions to immediately default to looking into self-healing or spam shielding champions. Just clicking the Tank tag is not going to just bring up all the most durable champions. You can notice the difference in durability between Tanks in-game a ton based on the tools they have:

Has High Self-Healing/Spam Shielding and Is Tank-

Almost every champion listed here are usually regarded as extremely durable champions, with the exception of Volibear since his has a decent cool down and he doesn't build all out defenses to begin with (and it doesn't scale with levels). There are a few other very durable champions, like and , but they are an exception due to extremely unique mechanics they possess that are never given to other champions and likely won't ever be again (long duration damage reduction and INFINITE health).

TL;DR If you want a durable champion, pick a champion that's effective health pool isn't limited to how high your max health, armor, and magic resist are. Edit: Note that I'm not saying every champion with a heal is the most durable, but the champions with a heal that also build defensively as their aim.