This question comes up a lot.
99% isopropyl alcohol works great.
Personally, I use LA's Totally Awesome, aka "LATA". You can pick it up at the dollar store. It will not damage the plastic. Works great and the price is right!
Here's what I do:
Get a plastic container with a lid (Sterilite, Rubbermaid, or something from the recycling bin like a pasta sauce jar, or a yogurt/cottage cheese/sour cream container/etc.).
Put your minis in the container, and fill it up with the LATA so that the minis are covered. Do not dilute the LATA with water. Put the lid on.
Soak for 24hrs.
Put on gloves, grab an old tooth brush, and scrub until you see bare plastic/metal.
Rinse, repeat if needed.
Make sure you wear gloves when scrubbing any cleaning agents off of your miniatures. The primary ingredient in a lot of these cleaning agents is a strong alcohol, which will also strip the natural oils from your skin. If you don't wear gloves, your hands will dry out and your skin will crack. It's unpleasant. Just wear gloves. And with most household cleaners, don't get it in your eyes or ingest it.
I've also found that if you trim the bristles on your old toothbrush short, they will be stiffer and help to get the paint off a little easier.
Good luck!