My recommendation are animals. I undertook investigations on wild mice and fish. These animals are very good bioindicator. Morphological examination of liver gives the best results in this case. You will find many examples of this method in my publications (certainly, see my ResearchGate). I studied effect of heavy metals on environment.

If you want to use a commonly used organism take E.fetida as Suresh said. If you want to have fairly realistic results then take a worm species that is common in your area (or where the samples are from). Or maybe springtails. They are also quite easy to handle.

I would definitely use earthworms to evaluate the impact of heavy metal contamination on soils! Worms live in the soil and also eat it, and so, they make perfect bio indicators. If you don't have any worms available, use Mice to perform a ecotoxicological approach. look into the liver and kidneys for the best analysis. you can always send dryes mice anatomical parts, to heavy metal analysis, as this will give o bioacumulation pattern.

I agree with my colleagues, earthworm is best for monitoring soil, then mice. Fish will be not be as good as these because here you you should concern with water. Likewise any plants grow in the study area can serve as a good indicator and analysis is little different than animals.

Earhworm (E. fetida), Collembola (Folsomia candida), Isopod (Porcellio scaber), Nematodes (Caenorhabditis elegans). These species are common used in soil ecotoxicology.

As already mentioned soil nematodes are a best bet for ecotoxicology of terrestrial ecosystems.

The reason is that they engage in bioturbation i.e. take in soil particles extract organic matter to which the metals re likely bound to and release gleaned soil particles. Thus measuring their accumulated metal content easily reflects metal pollution.

What i can tell you is to use not one bioassay only, but three at least, choice between the organisms that the other collegues suggest to you. I agree with them to use earthworm and collembola as standard bioassay but if you can utilize a biomarker in your battery i think you have more information on heavy metals risk assessment.