What would be an internet slang adjective to describe something perfect, mythical and ultimately kind

Solution 1:

As slang, something that's both better than normal and welcome could be described as the bee's knees:

[Merriam-Webster]

: a highly admired person or thing : CAT'S MEOW

Note the equivalent the cat's meow.

The Phrase Finder says this:

If something is said to be the bee's knees it is excellent - the highest quality.

It also mentions the cat's pyjamas and others.

A final piece of slang that I hear to describe quality is top drawer:

[Collins]

adjective
If you describe someone or something as top-drawer, you are saying, often in a humorous way, that they have a high social standing or are of very good quality.

(in British) noun
people of the highest standing, esp socially
(esp in the phrase out of the top drawer)

The players were top drawer.
The Sun (2012)

The fans will see what a top-drawer player he is.
The Sun (2009)

I can't think of anything that also has connotations of the mystical, however. Unless you think of those expressions involving animals as being mystical in some way.

Solution 2:

Unicorn if you mean something mythically perfect, unatainable. Named for the rare creature in mythology.

A Unicorn can refer to a man or a woman and is often used to describe the perfect catch or perfect partner. A Unicorn is a mythical creature, someone amazing who is hard to catch or simply a very rare find.

Urban Dictionary

Note, when I went to look for sources, I learned that unicorn has specific usage in certain communities concerning the perfect (almost always) female partner to join an established male-female couple in a relationship.