Re: What's the most transparent yellow?
I highly recommend Winsor & Newton's "Transparent Yellow 653". It says it is an "AZO Condensation", and its ID is PY128. I did a plot of it on a color wheel, using a color densitometer, and found it to be very close to optimum, true, scientific, "Yellow" in terms of being a primary color.
I use it almost exclusively for my flower paintings. It mixes with other transparent colors, to create deep, dark colors, and with white to create some very brilliant, light colors.
The only other Brand that I've found that offers that same, transparent pigment in oil paint is Rembrandt, who also has a PY128 pigment, although I've never tried it.
There are other transparent "Yellows", such as "Indian Yellow", but I haven't found one that plots, scientifically, on the color wheel as a primary yellow, as close as W & N Transparent Yellow 653 does.
If your desire is a transparent, Primary Yellow, this may be it. I use it, and it's been working well for me for quite a long time.