[sassan]

Though AV is one of the two settings most frequently used by me (The other being Manual), and probably many will agree with you that is the setting that they use very frequently, its hard to say that it is "the Best" setting as every setting has its own pros and cons and time to be used or not to.

The AV works fine as long as you have enough light and DOF is your main concern or you
have a manual lens adapted on a modern camera body and need to have some auto control for exposure.

I still use Tv or S or shutter priority, when I need to freeze a fast moving object and know the rough number that stops or shows the action at best.
Use P when am to busy with composition and don't want to be bother too much with other things.
I almost never use Full Auto (The green) that can give you actually very good picture in 95 percent of times.
And of course M, possibly for most of time as I can control all, the way I want it.

If you have a newer mirrorless camera with EVF (Electronic View Finder) you basically see all the setting effects on real time, via view finder but with usual DSLRs (Optical view finder) you need to take the image, then see the result in "Preview" mode.

As for WB or White balance, your best bet is to leave it on AWB (Auto White Balance), almost all the time. The common exceptions would be a very biased color light source such as dominance of Tungsten light (Too yellow)/ LED lights with high Kelvin number/Fluorescent light(Too white)/ deep sky objects or when using IR. I doubt you need to change the AWB at all.

As for ISO, your aim to use the lowest number that allow you to take an image with no shake (Slow shutter speed) causing blur. The higher the ISO, the more grain you have on your image and the faster your shutter.

If ever in doubt about any area, you can use "Auto Bracketing" that most cameras have it for exposure, many newer ones have for multiple other options including WB etc.

Best is to be your own critique after each shooting session and see what you are missing on images or what is bothering you in most images, then try to rectify them via the right setting.

Its all too easy if you put them into work. Have fun.