We report a record low thermal conductivity in polycrystalline MoS2 obtained by varying grain sizes and orientations in ultrathin films. By optimizing the sulphurisation parameters of nanometre-thick Mo layer, we could grow MoS2 films with tuneable morphologies. The values of thermal conductivity are extracted from a Raman laser power-dependent studies on suspended samples. The lowest value of thermal conductivity of 0.27 Wm-1K-1, which reaches a similar value as teflon, is obtained in a polycrystalline sample formed by a combination of horizontally and vertically oriented grains, with respect to bulk (001) monocrystal, in similar proportion. An analysis by means of molecular dynamics and finite element method simulations confirm that such situation leads to lower grain boundary conductance. We discuss the possible use of such thermal insulating films in the context of electronics and thermoelectricity.