By popular demand, this is the modpack I am using for my newest YouTube Let's Play series.
I tried to include a lot of mods that I haven't played with at all, or not much, in the past. There are mods to build your NPC empire, lot's of exploration and adventuring, but also lots of Magic and Tech mods. The world feels very alive with many added creatures, challenges, and fauna. You will find yourself exploring for many hours before settling on a base location because around every corner you will find new jaw dropping areas. There are also a few extra dimensions to explore with many dangers and rewards.
This modpack is quite heavy and I recommend you give it 4GB of RAM at a minimum. I personally run it with 6GB.