The impression material with lowest viscosity is
Impression plaster is a β-calcium sulfate hemihydrate used at a water/powder ratio of approximately 0.5 to 0.6. Its fluidity makes it suitable for making impressions of soft tissues in the uncompressed state, a characteristic of mucostatic impression material.
Ref: Phillips 12 ed page no 177
Plaster of paris
Agar-agar
Zinc oxide eugenol
Elastomers