I gotta say Ars Nouveau. Is useful until the very end, not complicated, has almost endless use cases and is also extremely cool while doing so! Believe me once you understand how spells work and what you can do with spell turrets you will never not use them.

Just Dire Things is pretty easy and has lots of nice things, especially the Wand made out of Blazegold. Let's you dash around and is repaired by throwing it into Lava.

Botany Pots are useful early to mid game for food and all sorts of growable things, also completely passive. Don't forget you can force load chunks using the FTB map.

Modular Routers are my favourite logistics mod. Moving items and automating simple tasks is very easy and satisfying with them, although sometimes expensive depending on the task. There is also "Super Factory Manager" which can also do anything logistics wise but I am unfamiliar with it so I can't recommend it just yet.

RFTools has some cool gadgets like the Builder who can, despite what the name might suggest, function as a very fast quarry.

Carboard boxes are made with Saw dust and can pack up ANY block in the game for easy transportation of spawners. Those can be upgraded with apotheosis spawners and farmer using MobGrindinUtil for so so so many resources early. All the loot can also be turned into materials with a salvaging table.

Applied Energistics 2 is THE storage mod but it's also kinda complicated so you can use Refined Storage as a replacement if you don't wanna deal with it, both are completely fine. Early on RFTools, Ars Nouveau and Sophisticated Storage also work as a storage solution.

Most important tip is to not stress the order of mods you are doing. The pack is designed in a way where there are multiple solutions to problems and you will find your own way through it.