Solgaleo is a bad restricted Pokemon in Regulation I and should probably not be used over options like Ho-Oh and Lunala. If you ignore this advice, some Tera Types are less bad than others.
Dark: Removes Solgaleo's Ghost and Dark weaknesses, which is relevant against Calyrex-Shadow, Flutter Mane, and Chien-Pao.
Water: Gets rid of the Fire weakness, making Koraidon, Incineroar, and Chi-Yu more tolerable; also resists Kyogre's Water Spout.
Fairy: Makes it marginally easier to address Miraidon's Draco Meteor.
Other Tera Types like Ground and Flying might be relevant depending on what your team needs. If you must use Solgaleo, it is better suited to a defensive Tera Type than an offensive one. Though I don't think you should use this Pokemon in the first place.
