The meta generally favors Pokemon with high stats, good Abilities, and good movepools. While the former two traits are self-explanatory, a natural question to ask is what constitutes a "good movepool." Well, it's as many as possible of
>9 BAP STAB-boosted Moves,
>10 BAP coverage Moves, hazards/removal, Knock Off, self-switching Moves, phazing Moves, non-Swagger Status conditions, Screens, D/E Moves, and Moves that can swing a matchup when ordering second (e.g. Counter/Mirror Coat/Bide/Metal Burst, Type-changing Moves, or Pain Split). This is partially due to Gen IX's stronger promotion of support Pokemon, but many of these tools are traditional characteristics of good Pokemon and can significantly influence a Pokemon's viability; for instance, ordering first against
Alolan Sandslash can be tougher than ordering first against
Glastrier despite their notable disparity in stats and coverage because the former wields a full suite of damage-returning Moves alongside Dig, Aurora Veil, and Snow Cloak, while the latter basically just learns attack moves.
The importance of Types is unavoidable, though. Historically, our game has been centralized around the Fairy-Fighting-Steel triangle, as these typings tend to appear on Pokemon with at least one—and often at least two—of the highly desirable attributes of extreme power, high survivability, and deep movepools. If we were discussing Gen VIII BBP, I would tell you that
Mega Gardevoir,
Equilibra, and
Conkeldurr were the three best Pokemon in the game, as they each exemplified these ideas excellently: Mega Gardevoir was arguably unparalleled from Gen VI through Gen VIII due to its good defensive typing, high Special Attack Rank, strong Abilities, and "perfect" movepool; Equilibra had excellent bulk, high-BAP STAB-boosted attacks, and the ability to escape and reverse practically any unfavorable matchup when ordering second through the usage of Doom Desire and Pain Split; and Conkeldurr could 4HKO the entire metagame while boasting above-average bulk, hazard and Item control, and strong priority. Amusingly, Gardevoir and Conkeldurr were comparative rarities in Gen VIII matches despite appearing on many players' profiles, which leads me to believe that most of the playerbase collectively made a gentlemen's agreement to decentralize the game and use different Pokemon, but I digress. All three are still great, and Gardevoir was actually buffed significantly by recent changes to status moves, but the Gen IX metagame isn't quite stable enough to make such assessments again.
This brings us to the anti-meta Types. Other popular Pokemon tended to possess a Poison, Psychic, or Ground typing, as each granted its user valuable, super-effective STAB-boosted attacks against the aforementioned meta triangle without incurring a weakness to any of its Types. These Pokemon would again be selected by the principles that we've already mentioned; this is the domain of your
Mega Garchomp, your
/
(Mega) Gengar, and your
/
(Mega) Slowbro. For more eclectic options, I personally enjoy and endorse a good
Smokomodo, and I've seen Pokemon like
Swampert and
Alolan Raichu rise in usage over this and the last Generation.
Finally, we have the "new meta" Types. As Maxim mentioned, Dragon-types are positioned well at the start of this generation;
Dragonite and
Hisuian Goodra are immediate examples, while CAPs like
Astrolotl and
Cyclohm meld powerful attacking potential with useful support movepools. Moreover, certain skilled players like
S0L1D G0LD seem to think Normal-types are strong in Gen IX, and it's hard to argue against the stats, movepools, and Abilities of Pokemon like
Snorlax,
Ursaluna, and
Mega Kangaskhan. We conclude with Grass-types, which are natural beneficiaries of Gen IX's support-oriented rebalancing efforts and have the convenient ability to handle both Ground-types and the increasingly common Water-types; solid offerings from this category include
Pyroak,
Caribolt, and
Tangrowth.
As for Items: Ring Target invalidates many offensive and defensive tactics, while Life Orb—the king of offensive Items—is the most broadly applicable damage-boosting option available and has carried two entire Generations worth of Pokemon through their relevant damage calculations. I'd start with those.