Some suggestions, snipped due to sheer size. If you'd like to read my rambling, please feel free.
In terms of raw information about the packs:
Mindcrack and Ultimate have the most mods, but TrainCraft and EE3 can be a bit of an issue when switching between the two.
Beta Pack A and Retro SSP are vintage packs with outdated mod versions and experiences.
Direwolf20 and YogCraft (FTB Lite, to some extent, as well) are easier to work with due to sheer size and lack of GregTech, but have current versions of the mods.
Tech World and Magic World are more focused on tech/magic mods respectively, although you'll find a bit of magic in tech and vice versa.
The various third-party packs, which are defined by being not produced by the FTB team, offer very different experiences from pack to pack depending on which one you choose. Voxel and Brony are more focused on giving more tools for Creative-mode servers, so they're not good for actual gameplay. They differ greatly from the main thread (lots of tech and a bit of magic) that FTB Team's packs offer.
I say try the ultimate pack it has most/all the mods in it then as you play more you can see what ones you dont like.
it was how i started in ftb
I wouldn't recommend Ultimate for a beginner, personally, as it has a lot of mods and can be quite overwhelming due to sheer size.
One last note: the size of a pack is proportional to its load time. No exceptions.