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.
Nice! Back in high school I played a modded server with my friend and we built a big solar panel farm and a nuclear reactor. That was a lot of fun! I want to say it was B-team, but it doesn’t seem to have a nuclear mod so it must have been something else. Playing with him was nice because he was a lot better at building than me lol.