I'm surprised no one has said Taric yet. You can peel with a stun that has a max range of 1875 (almost cait's ult at level 6). He can heal and shield them. He can body block skill shots because he is relatively tanky. He gives his ADC bonus armor; usually about 50 late game. He can make his ADC invincible for 2.5 seconds.
I think the biggest advantage that Taric has over every other ADC is that he can support from almost a full screen away while he has 100 hp. He can still use every ability he has from that range. If I almost die in a fight but the fight is still going, I don't have to leave. I can stay way in the back and support from the link on his W. This can be clearly seen after the fight at 21:30 in this video. I was almost dead but I was hanging back ready to support Twitch in case Jax wanted to jump on him. Again at 27:50, I link to Lee Sin, stand in the way of Jinx so she can't ult him, then I ult Lee sin and go a different way around in the jungle to get my ult on Viktor. At the end, Lee Sin had 4 hp left. He would have died but I was supporting him from a whole screen width away.
https://youtu.be/lg5xjOPOyaI
Also, there is a literal leash between Taric and his partner if that is what you are going for.