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 been enjoying Create a lot, partially because it’s giving me flashbacks to (one of?) the first huge mods Minecraft had: Better Than Wolves. Besides that ComputerCraft is one that I keep coming back to. I’ve once made a world where I started with a mining turtle and an unbreakable pickaxe that could only remove ComputerCraft things. All the resource gathering and building basically had to be done using Turtles.