Old Karma had the strongest shield in the game (280 + 0.8 AP I believe). With the changes coming in patch 5.10, Karma, with fully leveled E and R, will now give a shield with strength 410 + 0.8 AP on her primary target.
The next strongest shield is Janna's, at 240 + 0.7 AP.
At 737 AP, Karma's mantra E now gives a 1000 HP shield. Half of that to all nearby allies.
1000 . HP.
Kouga7 votesWhen was the last time you saw a Karma with 700+ AP that wasn't pubstomping?
Karma 410 + 0.8 AP
Shen: 850 (+ 135% AP)
If Shen had 737 AP, his shield would be 1,845 HP.
#CaptainShenMacia4Life
Sugar Momma2 votesFor all intents in purposes, 
's shield doesn't exactly count bc of it's huge cd and the fact that he never builds AP. If he did, then yes that's the strongest shield in the game.
Kouga1 votesWhy doesn't he count if Karma building Full AP Assassin build counts?
Besides, have you not heard the legend of Shenmacia?
If you're just talking about supports with Shields..
Then there is a lot more to consider than just numerical value.
Morgana's shield can negate disables on spells. Janna's shield gives AD to the carry which can tilt the scales in a trade. She can also shield turrets.
Yeah Karma has the "biggest" "support" "shield" but its just a shield.
Sugar Momma2 votesKarma "building full AP assassin?"
Karma's standard build for mid-top lane, and even support, is just like any other AP carry. She can easily reach those levels of AP late game, while Shen will never buy more than one AP item outside of URF.
Karma's shield gives a large speed boost, so it's not "just" a shield. However, because it gives no other effects, I think it's fair it's the biggest shield.
Marthian2 votesand she used her ult for it. Goodbye sustain, goodbye more burst.
Sugar Momma4 votesHer mantra - w is not used "for sustain" since it gives you back % MISSING hp. You don't use it to top off your health, you use it to survive all-ins.
Her mantra - E now allows her to save allies from all-ins, like other protective supports like 
and 
.
FantasySniper1 votesWhat "more burst"?
If you're going for damage, you're using her Soulflare.
jDVL20eCYs2 votesyes karma will be top tier next patch
Sugar Momma1 votesBut hopefully not top played D:
jDVL20eCYs2 votesdon't worry i play ekko only
Lugg2 votesGood, she needed some love.
dioxyide2 voteskarma main here just lookin at all this and i have to say karma is tricky to play but if you get farmed and build death cap and ludens then you 1 shot adcs not to mention her w does half of people hp now so if you screw up against a karma its usually going to mean death. and remember dont try to run and dont try to chase a karma it will not end well
jmonster1291 votesGOOD
Sugar Momma1 votesIkr?
coeurloyal1 votesShe's now going to have the longest root in the game, as well. It's a brilliant horizon for those of us that like to play Karma :)
Sugar Momma1 votesI'm excited for these changes!
ImFlutterShy1 votesAnd its also on a 3.6 CD with 40% cdr and lasts 5 seconds. It's just a better Riven shield which can be used on allies.
Sugar Momma1 votesThis is the MANTRA shield not the standard shield (which is being nerfed slightly). And it lasts four seconds :)
61413860_DEL1 votesI don't play karma a lot but I do love that shield :D but have you tried her top/mid? She's great and not extremely extremely squishy
Sugar Momma1 votesI've played over 300 games with her this season, top/mid/support and sometimes jungle :)
61413860_DEL1 votesOoh does karma jungle actually work?
Sugar Momma1 votesIt does! It'll be a lot easier with the change to her W coming next patch, but it's possible even now.
coeurloyal1 votesIt's not as spectacular as it sounds, and it's pretty risky with a bunch of tank jg's running around. But it's viable, and it's pretty fun.
Sugar Momma1 votes
is not the best jungler, I agree, but she does offer a lot. She's kind of like 
but she has more damage and utility at the cost of less objective control. She can build tank or damage as your team needs. Her ganks are also a force to be feared (speed up, targeted snare, slow).
The change this patch will help her sustain in the jg AND her clear time, especially early.