From easiest to hardest, the Tier 1 decks IMO:
1. Blacephalon: really straightforward. Build your bench by using search cards/discarding Fire Energy. Use Mind Blown as your main attack. Use Naganadel to grab discarded energy/can do extra damage when you have 3 Prize Cards. Use Beast Ring when your opponent has 3/4 Prizes left.
2a. Pikarom - there are two variants, the "straight one" where your strategy is mainly to get 6 energy on Pikarom and use Tag Bolt GX. The "toolbox variant" is harder to play where you juggle mainly between Zapdos/Jolteon/Pikarom as the matchup requirements. Both variants can be plus or minus Jirachi (nowadays, mostly plus)
2b. Zapdos - Use Jirachi as a draw engine. Keep poking your opponent with Zapdos's Thunderous Assault + electropower until they lose. The Ultra Beasts variant (Buzzwole and Nihlego) is most popular right now.
3. Zororoc - A fairly hated deck, but it is not brainless as people say it is. Use Zoroark GX Trade as your main draw engine. Lycanroc-GX is a free enemy Pokemon switch + a nice GX attack. You run a lot of different cards, so you need to consider what to discard/what you need to draw to win.
4. Ultra Malamar - Use Malamar to charge up your Pokemon. Giratina is your main direct damage attacker. Use Ultra Necrozma-GX's attack to finish off your opponent. I think you could argue this is easier/on par with Zororoc, but I choose to say it's hardest because of the nonintuitive setup of Ultra Necrozma
Happy to be corrected by more veteran folks.