Who are the best supports right now?

Janna is always a safe pick.

Nautilus the tanky CC machine has his fair usage.

Alistair is seeing his share now though, and is rapidly rising again.

Thresh is incredible depending on your entire team, but I feel his reliance on his skill cap and team in general puts him in a place where you would want to use him when going in with a team or a duo partner imo, otherwise he doesn't seem reliable/consistent enough.

I would say Leona falls into situational depending on your ADC but works with most relevant ones regardless.

Arches,

Kayle is underrated support. She works best with hypercarries like Kata, Yi, Kog, Azir, Cassio.
Morgana is actually pretty bad, if you are playing her for her Q, because you will probably not land it in high elo, unless you are scripting dog. She is just a good pick during laning vs CC bots like Nautilus, Leona, Thresh, Alistar, Blitz. Nami is actually better in TFs than Janna, but you have to actually land abilities. Janna is easy mode, Lulu loses lane, Sona wins lane, but is not very good later, Braum has been always a good pick,