If English wasn’t my first language but is my best language is it considered my first language?

I think "first language" would refer to your native language since you learned it first.  I would be proud of that fact.  Be sure to use your native language so you don't lose it.  I would also teach it to my children.

English words are defined in context. The same word means different things by words around it and intent understood. "First" is about numbering. It can mean time sequence. It can mean order of a list. It can mean performance in contest or ranking. It can mean preference."First language" can mean any of these things also, when put in the right context. Most times people asking, assume it as "Personal Preference".

The first language in North America was one of the Native American ones. (Time sequence)

French, Italian, German  First language is French (listed order)

Mandarin Chinese is first language for native speakers on the planet (ranked by number of people)

English is first language for global business contracts and dealings (ranked by preference)

No, but you're fluent in it.

Whatever you like.

There’s no hard rule about it. If you’re more fluent and confident in English then it’s logical to call it your first (choice) of language. If others choose to take that as being the first language you learned then that’s a harmless misunderstanding.

If someone asked you what your mother tongue was, then that’s not English. But if asked, I’d suggest that you always add that you’re not fluent and confident in that language as you’ve been immersed in English for most of your life.