Okta has been plagued by security issues [1], never heard of Ping Identity, Azure only makes sense if you get a sweetheart deal and are willing to deal with Azure's crap, and I'd never recommend anyone to use anything Google any more.

[1] https://www.flyingpenguin.com/?p=54722

Ping Identity run the largest enterprise identity platforms on the planet after merging with ForgeRock last year. Think HSBC, JP Morgan Chase-scale.

Ping is one of the oldest players in the business, they were founded in 2002 and had one of the earliest identity PaaS in the market (at least as far back as 2012). Haven't used their products much though.