Forestry.
I just enjoyed the whole concept of it. It is one of the very few mods that goes into agriculture instead of the more industrial side of Minecraft. I really enjoyed being able to automate farms with one to three blocks, a bit of power, and some items. I enjoyed turning the somewhat useless crops and agricultural items like sugarcane, saplings, seeds, and many more into something useful like power. I am very disappointed that the old farms are being removed and replaced with MultiBlock farms. I think the reason a lot of people liked the old farm was because it was simple to set up. I am not going to lie, it was a bit overpowered but I would rather they increase the power intake on the farming machines instead of introducing a new and more complex machine. It also added Bees. Although default forestry bees are kind of boring, once you add Extrabees, it becomes very interesting. The idea that you can mass produce almost any item you want with two bees is amazing. The ability to genetically manipulate bees and make super bees make you feel like a scientist and is great. The whole idea is very refreshing, and something i have never seen before. I hope they continue to add more and more stuff to the mod.