I think my favorite is Sigma because his shield, absorption, and stun are all helpful in keeping himself from taking damage so I can focus on healing the rest of the team, and they can also be used to help us secure kills like placing his shield in front of supports to cut off heals, blocking a Widowmaker, using his stun to negate ults, etc. Plus I like that Sigma players usually seem keen to slowly but steadily push forward and stay with the group rather than run ahead out of range chasing kills.
Least favorite would probably be Doomfist. A good Doom is awesome and self reliant but I still find them often off by themselves, out of range of heals, while the enemy team is attacking the dps and supports and my attention is split so I usually ignore Doom and assume he’ll come back for heals but if he dies or we die it doesn’t much matter.
Great tanks are rein or sigma for me when playing support. The ones that gave me the most bad experiences are doom and ball, i never saw a good doom in my team who isn’t suicidal and i never saw a ball that doesn’t need heals every second. I was told the ball doesn’t rely on heals but that’s all the balls that i got, they relied HARD on heals