Great Buildings (GB) are a main building type, and unlike all the other basic building types, these can be leveled using Forge Points (FP) to become even more powerful. I have blueprints for 27 different GB already (including 4 complete sets), and have no single blueprint yet for Obs and ToR - it looks like these two will be very hard to get soon. LoA - very useful for good production and for supply boost. Colo is useless - you can fight (you have to fight anyway) and use special buildings that have higher medal production than Colo without sucking valuable FP from you. ToB - I would not waste resources on it unless you want to stay in BA for very long - it is big, provides very small population for its size, and its production is much worse than LoA. SoZ is very important even if you are not fighter by nature - you need to attack other provinces and to complete fighting challenges if you want to progress. If I was starting from scratch, I would not waste FP on it (I am at level 2, and will get rid of it without regrets when I need space). OoD become pretty much useless very fast, even at IA stage - special buildings add to happiness and produce supplies much better without sucking valuable FP from you.

I happened to join during event, am active in quests, challenges, on Continent Map, and in guild hopping, and already own many event buildings (most of those are small, target happiness, and are useless though).

I am beginner, completing the IA, and here is my take on some GB from IA resident point of view.

Forge of empires forum beginners guide