Well, I've just finished the final chapter.
My favorite is Dragon abbey - that on tap mana is fantastic. Need 2m mana, burn lots of those culture boosting spells. Oh and mage damage boost? What is not to like?
#2 is Temple of the toads. Free heavy missile and heavy missile damage bonus. Way To Go. That said, I am human, and I understand elves don't have such a nice heavy missile unit.
#3: Blooming Trader guild. The 40% bonus for portal production just means I spend less time worrying about goods in chapters. Especially with less guest races having capped goods, the production bonus is really, really helpful. And for capped goods, heck, all those Portal Profit spells come in handy if I need them.
#4: I'm currently really liking the Thermal Springs of Youth. In terms of space efficiency this saves a huge amount of ground as it makes my houses more efficient, and doesn't requite culture to support it. Heck, it even provides culture.
#5: questionable here, but I think I'll opt for the Pyramid of Purification. Free sentient goods is very handy. Especially in chapter XVII where your trader isn't making acorns for you any more