For steel or titanium frames pretty much any commercial paint stripper will do. For aluminum frames, be very careful not to use a caustic (aka lye or sodium hydroxide) based stripper as it will attack the aluminum too. For a carbon frame, you are on your own unless the stripper specifically says it's safe for composites.

BTW, I never understood painting a Ti frame. Years ago Litespeed offered painted frames as an expensive upcharge that added weight. Why?