Best case is likely putting it back in the ground. Burning it results in a release of it's stored carbon to the atmosphere. Still not a perfect solution, since

Best case is likely putting it back in the ground. Burning it results in a release of it's stored carbon to the atmosphere. Still not a perfect solution, since many plastics will leach harmful substances into the ground without robust containment measures, but of the outcomes realistically available to us, plastic ending up in properly-run landfills is probably the best case.