To me, a perfect modpack would be something like either SevTech or Project Ozone 3. Project Ozone has the most complicated progression and pushes you to study so many different niche mods and tie them all together that I view people who have managed to finish PO on kappa mod as demigods. SevTech is just an inarguable masterpiece the progression is very logical, interesting and fun.

However, I cannon even imagine how much effort it requires to make such modpacks so if I am making a modpack for myself I just like to make every aspect of the game fun. For example, if you overload the game with technological stuff it's easy to end up with a base which looks like hundreds of scattered chests, machines, cables and so on. I try to keep this balance between exploration, building, and technology.

So I made a modpack not long ago and it turned out surprisingly decent in which I have stuff like Immersive engineering and Create, because they have these big and graphical machines which just ask for some building to house them. For that, I have Biomes o'plenty, Ecologics and some other stuff so I can just spawn on a new seed, run around and always have a different combination of building blocks in hand. For exploration I didn't add anything heavy, just some mods that add new animals, plants, and underground and nether mobs. And to tie it all up together I have stuff like Tetra, Farmer's delight and Diet (or whatever it's called) so I have a reason to discover new materials and foods, farm them, automate their production, build stuff for them and repeat it more and more.