My favorite one is surely Aether mod which essentially adds a sky land dimension(like Nether / End but not lazy) which doesn't affect the standard world at all but once you go there practically the entire game changes (monsters, animals, materials, tools, equipment, world traversal,... just everything works differently), also there are dungeons with bosses and cool loot (and I do mean bosses and cool loot, not a stone hole with mob spawner and a saddle).
Then to spice up the normal world I used Millenaire mod which adds living villages to your world, as in actual villages with things that resemble people who do active work(farming, building, woodcutting, cooking, shopkeeps,...), can train you, will trade, and have jobs for you that will build up your rep in towns, also more you help them out with resources the faster their village will grow, it is a very very cool thing to have.
For a time I also had some mod that added kingdoms (not sure what it's called), anyway it put in simple villages and castles where you would do quests and eventually climb to the rank of king... that part was a bit odd.
But as you got to the top your world would get attacked by dark forces (spawned evil shrines topside that would start spreading chaos and burning down villages) so you continuously had to fight that back and save villagers, concept was good but with everything really buggy it wasn't great to play.
Sadly with updates all these break constantly and almost never work at the same time.