I am thinking ZIlean because he could go back in time and make all of the others never have existed.....
i chose
or 
Poppy is my protector.

Tahm outclases 'protectors' in terms of protecting his ally. No other ally who's MEANT to protect his ally can do it as well as Tahm can.
Thresh's lanturn is tremendously good too, BUT it won't stop someone who has already launched their attack and that attack is too strong even with the lantern shield.
So you may think "Well, Tahm can't stop damage over time affects" Well, no, alone he can't... but as a supp he should be building face of the mountain, so.. combining the shield with his W should be near absolute protection
At that point the most prominent flaw in his defense is your teammate prematurely exiting your stomach.
(and then theres blitz, thresh and other hookers...)
Tahm Kench or Zilean. Tahm protects, but Zilean can stop you from dying, so they're both good.
Shen. Even if I do get hit I can just slap somebody that he's stabbed recently and re-heal :D
I'd say Lollipoppy. She can destroy life by simply looking at it. I'd want her to protect me simply because none of the other champions can stop her rain of terror.
Zilian, he can stun the others to slow them down, speed you up to get away, and ult you if you are about to die.
poppy simple
Shen , 1: he'll protect you and change focus, Tahm Kench is a no because he tries to eat you for a reason... TO EAT YOU