Magic Academy, Builders and expansions are always the go-to's.
The Magic Academy just got a lot more attractive to upgrade now with the introduction of crafting. The higher your MA, the more spell fragments you get from disenchanting things, best bang for your buck right there.
Builders are good until you hit time-consuming chapters like Orcs. No need to buy 5 builders when they will be doing nothing for months when there is nothing to upgrade. But even in higher chapters, extra builders can be great, for a short time. They will run out of things to do.
Expansions are great, but get very costly, very quick. Buy what you can afford, but you can always gain free expansions from scouting.
Magic Residences and Workshops may look appealing, but they will quickly become outdated and expensive to upgrade unless you are in a 10-chest fellowship. If you do earn weekly blueprints, than you may want to consider how powerful these buildings are.
Never buy culture, you will outgrow it very quickly and they are not upgradeable by any means.