As a new player, what pack should I prioritize opening?

If you want functional deck then either abyss encounters or the new rampage of the forest have relatively cheap decks. Abyss gives aliens, SSA and Flying Fish while Rampage have sylvans. Those are just decks that aren't too reliant backrow staples, though flying fish does need some backrow.

The alien deck is very good from Abyssal Encounters. I spent only 500 gems and took me to decent gold rank in PvP. Not bad IMO as I was also a new player at the time

Go for Abyssal Encounters. The alien deck is a tier 1 deck and the cards are relatively easy to get. The deck is a great counter deck as you can take control of enemy monsters easily as well has destroy their spell/trap cards.

if u want to be pro like me that always reach KOG in day 1 or 2, u need to invest some of money, if not u can stay as f2p player and aim boxes that contain Cyber Angel deck support card like senju, saffira or sonic bird that way u can reach KOG but in a late day like 1 week before the end of the month since all pro or p2w player alr reach KOG so u didnt got much challenger, also if u reach legend rank when u against a player using a CA deck and he currently legend rank 3 and u legend rank 1 or 2, i recommened for u to copy their deck and use it to reach KOG 90% of my experiment success

If you're planning to be f2p,and you want to stay that way, never EVER buy boxes only buy miniboxes. I would recommend buying the new forest rampage one, because rn, it is the current meta, and you want to do your best to keep up with the meta. Also it comes with a staple card, Hey, Trunade! it also has nearly all the cards for a somewhat f2p deck Sylvan. Do some research (watch some pro youtubers) and learn how to play, and what cards to get. That's just my advice anyway :)

Depends on what your priority is. If you want to play competitively directly I suggest grabbing the box that you could build cheap and good meta decks with (e.g. Abyss Encounter with Aliens during the meta days) and climb the ladder grabbing some R and SR tickets along the way grab some staple such as Enemy Controller, Metalmorph which could help you improve some decks that you might want to build in the future.

land of the titans

Gladiator Beasts are from Galactic Origin not Generation Next. If you mean Arcana-5HD then yes. Just like first answerer said. There's no clear suggestion. but I recommend you just buy cards for farming rather than PvP. 4 first packs from Ultimate Rising-Flame of the Tyrant are good for farming. You can buy in Valkyrie's rage to get Unhappy Girl, Crimson Kingdom for Buster Blader and Dark Paladin also try to buy at Electric Overload to get Troop Dragon

Let me start off by saying that the current meta is arguably the healthiest it's ever been. There's no clear answer, so I'll give you a couple good ones.

As a new player who doesn't want to spend much at all on Duel Links you'd be well of going for the newest mini-box, Rampage of the Forest, while simultaneously saving up as many extra gems as you can for techs in other packs that will make your deck even stronger. Sylvans just launched and they fit the meta like a glove. You would also have a ton of fun with Aromages if you're willing to wait for the Card Trader to drop the essentials you need to build a complete Aromage deck.

If you want to spend actual money on the game, go for Abyss Encounters. It introduced Archfiends, Aliens, and Sea Stealth Attack; 3 top-tier archetypes. And if you ever get bored of one, you'll probably have the cards to build one of the other two since they're all in the same pack.

P.S.: Use your reward tickets/ranked tickets on useful tech cards you can't get anywhere else like Anti Magic Arrows for example.

It depends, if you want boring but annoying and high win rate - choose Gladiator Beast from Generation Next box

if you want cheap but powerful f2p deck that can steal opponent monster, choose Alien from Abyss Encounter box

last but not least, if you want powerful and balanced all-around deck, try baboon deck from land of titans mini box