What buildings yield the most power increase for the building events?

Most buildings follow a similar pattern, the power increase with each upgrade starts very small and will get bigger with each level. Once you get your buildings up to around level 20 you will be getting large power awards with each upgrade.

Some buildings though do award more than others, these tend to be the specialised buildings that require specific resources to upgrade such as chips, power cores, war horns etc. in addition to the regular resources.

You can always check the power that you will get from each upgrade by tapping on the building and then on the 'Details' option. This will tell you what the benefits are of each upgrade and the power level it will have. You can see what each upgrade will take you too.