What is the best thing to use for thinning Tamiya/Poly Scale acrylics? I've heard several things (water, denatired alcohol, Windex, windshield washer fluid), but what do you use and in what ratio?

I can anwer for Tamiya, because I don't use Pollyscale.
I dilute those with:
- isopropylic alchool (not denaturated because messes up the color tones)
- windshield cleaning (read on the can the one made wit a mix of alchools)
- Tamiya thinner (silly enough answer)
For basecoats i dilute usually 50%-50% but for other uses even up to 90%-10% (thinner to color). Anyway it depends on the density of yhe color itself.
Ciao

I just did a basecoat using isopropyl alcohol and it came out great, but best of all, no clogs with my airbrush!!! Thanks for your help!!!

I've been using pollyscale quite extensively. I thin them with water (supposedly distilled water is best....I use tap water), a little isopropyl alchohol, and acrylic extender. The extender will slow the drying time a little and improve the flow which further reduces clogs in the ole brush. For tamiya I use just the alchohol and extender.

Do you thin for figure painting base coats?

Yes I do but I prime it first with standard white acrylic primer I find at arts craft stores that can be airbrushed. Once I've got it dry, I apply several coats of diluted Tamiya acrylics until I get the desired color density.

When I paint using a brush I thin Tamiya paints with water. When I airbrush I use Tamiya's paint thinner.

I use both IPA and water to thin thru AB.

The only add'l comment that I'll add to the other great responses is that I find thinning with IPA results in faster drying of the paint. Not a surprise, I suppose but just be aware of it...

Keith