Well, not to jump to the obvious answer too quickly, but the respective big boxes are a good place to start. O&C in particular revolutionizes virus play.
Jinteki has a lot of very different decks depending on what you want to do with them, but again the big box is a good place to start. (The core set leaves jinteki especially with big gaps to start off with)
The long and the short is that if you’re focusing heavily on certain factions, the big boxes are for you; it’s what they were designed for.
Oh, also, the pack with Jackson Howard in it. Get that one like, first.