Long term: publish everything that does not have a formal constraint. Practically the first dataset often exists as tabular data. Today used internally for reasons such as economical or statistical reports. Geographical data is often used and maintained for internal reasons. If you have routines and data deliveries in place, publishing is an easy task.
When you investigate what data you can publish, browse through your own website. Some of it can also be published as open data. It can be helpful to look at what data similar organizations publish.