The Meaning of Life, the Universe, and Everything.
Personally, I would say Advent of Ascension for 1.12.2. It adds hundreds of mobs, over 20 dimension, new weapons types, hundreds of items and many new interesting gameplay mechanics such as summoning. I have one complaint, though. It lags quite frequently for me. The sheer amount of content sometimes makes me turn the game off and on again due to the lag. Anyway, what's your favourite mod?
It's tough for me due to the mods I've used over time from mainstream to niche, from tech to other magic ideas, storage mods that I really like and so on, but I'd say LightningCraft for this post as it adds a bit of everything: a dimension, tools/armour/non tiered special swords, tinkers smeltery support, basic tech aspects in a new way from what I've seen (like using Hot Air or energy plants from ProdigyTech, LightningCraft being different in how you use Lightning to generate power compared to just a coal generator or solar all the time as the default method even though it is supported), utility items and blocks, new mobs and more. It's unfortunate we didn't get the planned Aether like dimension but instead an Underworld but it's still cool to me for it's ores, mobs, structures and ideas.
ArchitectureCraft is probably my favorite mod, as it adds in a really quick way to add enhanced levels of detail to your build that regular Minecraft just can't come anywhere close to. The mod also doesn't get bogged down by the extreme detail like a mod such as Chisels n Bits or Little Tiles does.