Depends on the sample parameters and the measurements you want to make and how you are making them.
Usually you want to be sure that almost all the applied voltage is dropped across your sample, so the metal has to have very low resistance in the temperature ranges you are using. It should also withstand the environment the experiment is conducted in and be available to you within your budget ... gold is commonly used for the metal, you may also have another kind of crystal whose properties are well known between your sample and the electrode.
Companies who prepare such samples usually will tell you what sort of contacts are available.