My favorite modpack is definitely the old Tekkit for Minecraft 1.5. I’ve probably spent more hours in Tekkit than any other Minecraft version or modpack.
As for a mod, I wouldn’t say this is my favorite mod, but I want to recommend Chalk. It’s a very simple mod that adds a chalk item, which you can use to draw arrows on the ground. It’s very useful for marking your path when exploring open areas and cave systems.
I’ve had quite some modded worlds and playthroughs, but the mod that always resonated with me the most was botania.
The way it sells itself as a plant magic mod, while being an automation tech mod at heart (but still requiring tinkering with redstone) is just lovely. It brought interesting ideas into the world, and the way you generate, manipulate and use these pools of mana (while also avoiding GUIs as much as possible) has always been so enchanting and fun to me. :D
It even contains a storage system (craftable once you visited the end), which is unlike any other centralized/universal storage system I’ve seen in other mods.