Head-to-head
Rayquaza vs Arceus
The sky-dragon emperor vs the Pokémon god.
Side-by-side
Rayquaza
Dragon/Flying · #0384
BST: 680
Arceus
Normal/None · #0493
BST: 720
Verdict
Winner: Mega Rayquaza (raw damage) / Arceus (with the right Plate)
Mega Rayquaza is the highest-BST non-Arceus Pokémon. Arceus can theoretically counter Rayquaza by picking Fairy or Ice Plate.
Context: why this matchup matters
Rayquaza (Gen 3) is the Dragon/Flying weather trio leader — the only Pokémon that can use Mega Evolution without a Mega Stone (just requires V-Create or any Dragon move). Arceus (Gen 4) is the literal 'Pokémon god' that created the universe in lore. Both are Ubers-tier mythicals.
Stat battle: who has the numbers?
Rayquaza: 105/150/90/150/90/95 BST 680. Mixed attacker with 150 Atk and 150 SpA. Arceus: 120/120/120/120/120/120 BST 720. Perfect balance — Arceus has the highest BST of any Pokémon currently (tied with Mega Mewtwo X/Y). Mega Rayquaza (780 BST: 105/180/100/180/100/115) is unrestricted Mega Evolution.
Arceus's signature mechanic: holds a Plate item to change type. With Earth Plate, Arceus becomes Ground-type with STAB Earthquake. With Dragon Plate, Arceus becomes Dragon-type, matching Rayquaza's typing. Effectively Arceus is 18 Pokémon in one.
Full matchup analysis
Mega Rayquaza vs Arceus-Normal: Mega Rayquaza wins. Outrage hits Arceus 1× neutral (Normal doesn't resist Dragon), and 180 Atk + Mega bonuses one-shots Arceus's defensive stats.
Mega Rayquaza vs Arceus-Fairy: Arceus wins. Fairy Plate makes Arceus immune to Dragon — Rayquaza's STAB Outrage does 0 damage. Arceus-Fairy's Judgment (signature move, BP 100, becomes Fairy with Fairy Plate) does 4× damage to Rayquaza (Fairy hits Dragon for 2× × Dragon weakness to Fairy 2× = 4×).
Mega Rayquaza vs Arceus-Ice: Arceus wins. Ice Plate Arceus's Judgment 4× hits Rayquaza (Dragon/Flying double-weak to Ice).
General answer: Rayquaza wins by raw power; Arceus wins by counter-typing. Arceus's Plate flexibility means it can pick the matchup that beats Rayquaza.
Smogon Anything Goes (AG) tier: Mega Rayquaza is THE tier-defining Pokémon — banned from Ubers, requires the AG metagame to legally use. Arceus-Normal/Fairy/Ground are common Ubers picks but pale in raw damage output next to Mega Rayquaza.
When to pick each
Pick Rayquaza for: Smogon AG tier (it's the meta-defining Pokémon), VGC restricted formats where Mega Stones are allowed, Dragon Ascent V-Create offense. Mega Rayquaza Delta Stream nullifies opposing weather AND prevents Flying super-effective damage.
Pick Arceus for: Smogon Ubers tier (Arceus-Normal/Fairy/Ground are top picks), Calm Mind + Judgment + Recover stall, support roles (Stealth Rock, Defog), Plate flexibility to counter specific threats. Arceus is the most balanced Pokémon ever designed — 720 BST evenly split means it can fill ANY role.