In first place we have Mega Rayquaza. There are a lot of reasons why this Pokémon is ranked #1 including stats, movepool, ability, and amazing balencing, so lets to through them one at a time.
Mega Rayquaza is tied for the highest base stat total of any usable Pokémon with the Mega Mewtwos. The only Pokémon with a higher base stat total is Eternamax Eternatus, which is unable to be used by the player. It's effective base stat total is 600 because of its 180 special attack stat, which is still extremely high. It has extremely good bulk as both of its defensive stats are 100 which is pretty good for a Pokémon to have just one, but both is really good, along with an HP stat of 105. Its speed stat is 115, which is above average, but is not the fastest thing in the world. It has 180 in both attacks, which makes it tied for the second best special attack and tied for the fourth best attack ever.
Mega Rayquaza has an extremely good moveset as it can use its great attack stat well with its moveset. Some of its best moves are listed below.
Swords Dance and Dragon Dance are great moves as they set up Mega Rayquaza to do more damage. Swords Dance doubles you attack stat while Dragon Dance multiplies your attack stat and speed stat by 1.5 times. Dragon Ascent is a 120 base power move that lowers your defenses. Extreme Speed is an 80 base power move that has +2 priority, meaning it will outspeed all other damaging moves. Earthquake is a 100 base power move that hits both targets, but also hits your ally. V-Create is the highest damage dealing move that doesn't make the user faint having a base power of 180 at the cost of lowering your defense, special defense, and speed, they balenced this out by making it the signature move of Victini and Rayquaza for some reason. These are already great moves but you can add extra damage with Swords Dance and Dragon Dance, making these already incredible damage dealing moves deal even more damage.
Mega Rayquaza is the mega evolution of Rayquaza, which has the ability Air Lock. Air Lock prevents the effects of all weathers, including the weathers from Primal Kyogre and Primal Groudon which normally cannot be overwritten by other weathers. When Rayquaza mega evolves, it gets Delta Stream, which is the third of the three primal weathers, so it can't be overwritten by normal weathers and it can overwrite the other two primal weathers, at the cost of Delta Stream being able to be overwritten by the other primal weathers. Delta Stream has the effect where if a flying type (such as Mega Rayquaza) is on the field, all of it's weaknesses are removed, but it still has resistances from the types it resists.
The Pokémon Company is great at balencing their creatures and making every Pokémon have a fair chance. They mega evolutions, some of the best Pokémon ever like Mega Charizard Y and Mega Kangaskhan. They thought the best way of balencing these creatures was to make them be forced to hold an item. When they made Mega Rayquaza, they decided to make it the ONLY MEGA EVOLUTION THAT DOESN'T NEED TO HOLD A MEGA STONE! This lets it be the only mega evolution that can hold items such as choice scarfs, choice bands, or life orbs to let it outspeed more Pokémon or deal more damage. This is the main reason why I put this over the #2 Pokémon as the #2 is forced to hold an item while Mega Rayquaza doesn't for some reason.
This Pokémon is so good, that Pokémon has given it the harshest nerf ever for any Pokémon. This Pokémon is in my opinion, the best attacking Pokémon ever, as it has a 180 base attack with a 180 base power move that it can use. It can also buff this attack with swords dance or dragon dance. If you don't feel like using a base 180 base power move, you can use extreme speed to always go first. In the end, they nerfed this Pokémon by removing it from Pokémon Sword and Pokémon Shield, the harshest nerf ever. At least until Legends Z-A.