I use either Polly Scale Easy Lift Off (ELO) which I think is branded as Testors now. Or I have used denatured alcohol in or out of a ultrasonic cleaner.
I have also heard of the brake fluid but that gets really oily and you need to be very good when cleaning the parts before repainting especially if you are using water based paint. I have heard of using Pine Sol but I think that may be the same issue. I have heard of using oven cleaner as well but that is very toxic and you need to be in a good ventilated area.
Most of the above are good on plastic. Some work better than others on different paints.
For metal, I have used lacquer thinner. The funny story with this is I wanted to strip paint off a matchbox car and repaint it. I was about 12 at time and my dad told me to use lacquer thinner. So I took a can filled it with thinner and dropped the car in. Well it took the paint off very good in a short time but all the plastic pieces were missing. I didn't realize that I needed to take it apart before I stripped it and just use the lacquer thinner on the metal pieces. Live and learn
It is what you learn, after you know it all, that counts.