What do you think I should aim for?

If I started playing today, I would go for Abyss Encounters (SSA, Golden Flying Fish, Alien) and Rampage of the Forest (Sylvan, Insect Queen, and the OP spell for Beatdown/CA decks - Hey, Trunade!).
But, honestly, you can build very strong decks with most boxes (not all, but most), it mostly depends on your taste.
For example, if you are aiming for a Cyber Angel deck, then you need to spend quite a lot of gems or cash in Galactic Origin and Neo-Impact boxes, since that's where you get Saffiras and Senjus for your ritual deck.

You know what flying fish is totally F2P u can get most of the cards from CT and others for exchange tickets and investing just a handful of gems in abyss encounter. They r really OP with their field spell and can easily beat Sylvans or most of the menace.
I use Sylvans but they don't stand a chance untill u get good cards to GY against flying fish. Flying fish are op and easy to use then Sylvans because they r not that complex with their effects atm. If u get SSA from box then u r guaranteed winning it. In my opinion u should rather invest in flying fish and aromage they r both f2p ATM and best decks.

Any tier one deck, honestly. Sylvans are strong, but they can be countered by CA pretty easily. That's not to say they aren't the strongest deck at the moment, (because they are) but they are uninteresting, and the community doesn't need more sylvan players. I'm assuming Konami might nerf them in the near future by either making some cards semi-limited or introducing new cards (most probable) that can counter them.

sylvans or aliens

If you can get all the cards, I would definitely recommend Sylvan. Strongest deck at the moment in my opinion