The best building to use RR's on is the one that will do you the most good if you upgrade it. What the most good is, depends on your play style and needs. If you need population and have a good population building you can upgrade for 1,000 population boost, that could be good unless you had a better population building that would give you 1100 pop boost when upgraded. Ditto for everything else. It's good if it helps and best if it helps the most.
AJ