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’m going to list some more mods I’ve found that I really like:
Better Harvesting - adds fast leaf decay, vein mining, and automatically replanting crops.
Ambient sounds - adds some much needed ambient noise to the game
Convenient mobGriefing - splits mobgGiefing into multiple gamerules, so you can disable bad mob griefing (like creepers, endermen) but keep OK mob griefing (villiagers, sheep, snow golems)
Comforts - adds sleeping bags so you can skip the night while exploring without setting your spawn.