I love Supernaturals

In English the show is just called Supernatural, like the adjective.

I most love Tthe TV show Supernaturals.

If you want stronger wording than just "love", you could say "I adore the TV show Supernatural" or even "I am obsessed with the TV show Supernatural."

This TV series iwas my most watchabled series fromduring my teenager time years (and early twenties).

I have changed this to past tense.

You don't really need the brackets here either, but I left them as I get you are trying to add a confessional ending to the sentence. To enhance this, you could phrase it as: "(and maybe my early twenties as well)." or "(and maybe also my early twenties)."

It have nears 15 seasons, but iI only watch onlyed 12, because I think that (spoiler) peace between God and The Darkness iswas the best ending there.

It has exactly 15 seasons, so no need to add "near".

In English she was referred to as "The Darkness".

You could also end the sentence as: "peace between God and The Darkness was the best ending possible".

Also I love the relationships between Sam, Dean and Castiel.

"Relationships" not relations. Also, it's a plural as there are multiple relationships, ie. Sam-Dean (brothers), Sam-Castiel (friends), Dean-Castiel (friends).

A small note is that saying 2 characters/people have relations or have had relations can be interpreted as a euphemism for sex. You haven't done this here, but it's something to watch out for in the future. This is a super common error for English learners.

They are the best Ttrio I watched in my entire TV serieshave ever seen on TV.

It's very common to say: "the best X I've ever seen."

No need to capitalise Trio here. Only capitalise proper nouns, which are the specific names of people, places, days/months, or organisations (eg. Jane, London, Monday/July, or the World Health Organisation).