The hardest support counter to Janna is Sona, who has a 50.23% winrate against her

Because she has no significant weaknesses while also being just above par in every significant area that a support should care about?

Janna is just good in every situation.

Roaming? Good movement speed and slows for that.

ADC protection? Gotta shield and movement speed for that.

ADC amplification? Got extra damage for that.

Anti-siege? Can shield turrets for that, and use a tornado on a big minion wave.

Not even considering her ultimate.

She's really strong. I can't find many weaknesses in her, besides maybe attrition. But that's really hard to pull off against her, too.

just look at IMT and you know why - she lets you get away with about everything due to her utility and the "run forest, run!" passive...in SoloQ often elading to overchasing and thus throwing the game away.