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.
A had a lot of fun recently with Tekkit 2, but I think that was mostly nostalgia.
I've also been playing quite a bit of AoF6 and the performance of Fabric has been absolutely stellar.
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.
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.
i like to keep it vanilla, by which i mean i want to be able to still play on vanilla servers. so my favourite mods are interactic, visuality, smooth swapping and others of this nature. making the game look better while keeping the gameplay unaffected
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.
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.
Me and my friend are playing a Create modpack and just discovered Tetra for the first time. It adds so much wonderful complexity to the tools and weapons in the game. It's honestly so much more streamlined then Tinkerer's Construct.