Best (Non-Shiny) Paint for Customs?

I use a lot of different brands but my go-to is Humbrol. I'll mix a matt and gloss until I get a desired sheen.

Another tip is I typically coat whatever I paint in Mod Podge, which comes in a matte or gloss. The matte pretty much puts up an invisible layer over whatever you painted but the gloss will tend to shine things up a bit. I sometimes mix these to get a custom finish.

The easiest thing you can do is buy sealers/varnishes in the finish you want. Then you can use whatever paint you want and simply use the sealer to change the finish.

Airbrushing also tends to give paint a matte finish. For example, if I brush on Formula P3 paint, they're satin, but if I airbrush them they're a lot more matte.

Also, saw you asking for ideas, how about making a Brainiac with that Beetle figure? The Spiderman and Cap figures are generic, you could make anything with them.