Definitely Millenaire, it turns Minecraft into both an Adventure game AND an RTS, I love pitted villages against each other and watching the raids to see who is victorious.
Plus it make finding a home base for everyday Minecrafting easy.
Then there are the lonebuildings, these are fun, scary too like when you are minding your business chopping down a forest when out of the blue a pack of bandits swarm you and you have to defend yourself lol