Primarily, I use acrylic paints. Most any acrylics will work fine, be they the $.99 specials or the high-end professional series. Acrylics mix easily, dry quickly, and react well with primer.
Primarily, I use the Ceramcoat brand of acryclics, commonly found in craft stores. I’ve used these since I was a kid. I know some customizers who swear a decent custom cannot be painted with these kinds of acrylics. I feel this site is a testament to the contrary, as everything you see was painted with the “cheap” paints.
I’ve dabbled in the more expensive brands, and found them to be fine paints. But there’s nothing I can do with them that I can’t do with Ceramcoat cheaper.
One additional thought: On rare occasions I’ll start applying a second coat, and the first coat will start to rub off as the second is applied. When that happens, I stop and shoot the figure with Testor’s Dullcote, and then continue painting without a problem. I’m told the more expensive acrylics don’t experience this.