The ideal polymer depend on the final target application, e.g. a dental injectable gel for regeenration e.g. bone, collagen, or drug release, or etc. what would be other components Neverthless, there are number of general parameters including the biocompatiblity of the polymer, rheology of gel, solubility/melting properties of the polymer, the possible reaction between polymer and other component of the gel, gelling time, the maximum force require for injection with different needles, etc. Please be specific about your target application and other compositions in the target gel matrix.
I agree with Amin, but above all this you need to be sure the gel is sterile and biocompatible and there is strong clinical support for the application.