I've used several different things...
1. Scalecoat - Works well on 1/55 and similar "smooth" surfaces. By rubbing, you can remove some paints in as little as five minutes, with just a heavily doused cotton swab. Just don't rub too hard or you will mar the surface.
2. GooGone - Same as scale coat.
3. Orange Floor Cleaner - Will strip paint off a 1/48 (or most anything) without discoloring or harming the plastic, given about 1 month in a OFC bath. Plus it leaves a nice scent.
4. Simple Green - Will strip the paint off anything given a long period of time (approx 1 month). Not only will it completely remove paint, it will not harm the plastic. Also, it will remove old stuck on paint on plastic handled paint brushes and return the bristles to normal, just be sure not to soak wooden handled brushes as the Simple Green will bloat the handle. Oh, and it leaves a nice scent.
Always remember wash off anything you use to remove paint before priming or the resutls could be disasterous.