The colloquial noun canon is masculine but generally refer to a woman.

Y’a un canon qui vient d’arriver !

When used as an adjective, canon (which is close to "hot") keeps the masculine form, might not mark the plural and can be used for both genders :

Elle est canon.

Il est canon.

Ils sont canon(s).

Despite matching another meaning of canon like in un canon de beauté (formal), I doubt they share the same origin. I suspect the power and potential "damages" this canon (cannon) can cause is a more plausible explanation. Another powerful device is a bomb which belongs to the same military vocabulary :

Cette fille, c’est une bombe !.

See also Usage of 'hot' meaning 'good-looking' or 'attractive'

Notes:

If you use meuf, chances are high you say c’est qui instead of qui c’est:

C’est qui, la meuf la plus canon du monde ?

and you might also use keum (although less common than meuf) instead of mec :

Le keum le plus canon que j’aie rencontré !

However, canon is being replaced by hot among the younger generation and verlan popularity is fading out so that might be:

Le mec le plus hot que j’aie rencontré !

Not only can it apply to men, but also to some objects or concepts !

Ex:

C’est vraiment un film canon !

Ce coureur a pris un départ canon.