Nail polish remover. Large bottle, costs peanuts, lasts ages, and is instant. Takes off all paint, and leaves the plastic perfect. Brilliant. The last bottle I bought was about $1.50 some months back. Usually use tissue soaked in it, then rub down the model. When tough, I use an old toothbrush, and 2-3 applications.
It has another application. It removes excess filler. On a recent build, where there's excess filler where the fuselage meets the fin, I used a cotton ear-bud, soaked in nail polish remover, and rubbed it along the seam. Took quite a while, to be honest, but the finish was better than sanding alone (I did some sanding).
In my view, along with Future/Klear/Pledge everyone needs a bottle of nail polish remover!