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.
Back In the good old days of Minecraft, Tekkit was great, as was Feed the Beast. A good friend and I had a server we would play on in college - he did everything with magic, and I did everything with technology. We had a pretty awesome base with a big wizard tower for him and a solar panel powered tech lab for me.