I'd like some informed opinion from keen tango dancers out there who do private lessons with any teachers in Sydney. Who do you think is the best teacher/s in your opinion? Also what is the average price range per 1 private lesson? Thanks in advance.

Your question is not simple to answer, firstly you must decide what style you wish to pursue, if you then find a competent teacher of that style you may not like them, if you do like them you must be willing to spend time developing basic skills, for every hour of private class you will need to practice for at least ten hours with a competent partner. Several years later you will begin to know what you need to know.
It is worth the effort the rewards are priceless.
David

I'm not new to tango, and can dance a variety of styles. I guess I'm looking for tango teachers in traditional tango salon style - which is the most prevalent style, at least here in Sydney. 

Here's a list of my "criteria"

1) Good mentors and communicators: 

I want teachers who can work with me on improvement of my dance quality at my personal pace, who are well informed on tango salon style and are sensitive to cultural aspects of this type of tango. 

2) Salon style syllabus:

And I'm not talking about "technique" per se as this comes with practice - as David G has mentioned. I can practice technique on my own. Teachers should be capable of intelligent analysis of my dance skills in order for me to be able to improve on my following skills, understanding musicality and salon style syllabus/figures structure, as well as styling.

3) Dance teaching education:

Teachers should be qualified dance teachers not someone who simply dances tango for many years.

If by salon style you mean V style, both heads looking in the same direction, I know of only one couple who teaches this.
If you require BsAs qualified teachers, again I know of only one couple so qualified, they teach close embrace, chest to chest, there may be others.
You can look on the various teachers webpages and find who is who.
I am not endorsing anyone or eliminating anyone, it is too complex for that.
Would be happy to discuss in person, see at a milong,
David