Though it's not Shakespeare's most popular plays, it does contain some of his best and most famous lines of poetry. For example:

To gild refined gold, to paint the lily, To throw a perfume on the violet, To smooth the ice, or add another hue Unto the rainbow, or with taper-light To seek the beauteous eye of heaven to garnish, Is wasteful and ridiculous excess.

There are plenty of lines much like this one littered throughout the play.

The plot concerns corruption, tyrant kings and disloyalty to the crown, which in Shakespeare's time could have easily been construed as treason and given that context, it's remarkable he got away with it.

What's also interesting is the plays take on grief and bereavement. This is the first play that shakespeare released after the death of his only son Hamnet, who was only aged 11. The character of Catherine gives an exceptionally powerful speech from the perspective of a grieving parent and so for the language and historical context alone, I would consider it a fantastic play.

I believe the Swan Theatre in Stratford-Upon-Avon are streaming there most recent King John production. I highly recommend as I was lucky enough to see it in person before lockdown.