How powerful a Pokemon is can be determined by many different factors. Their Base Stats determine how naturally strong a Pokemon can be but there are many categories that Pokemon fit into.
Normal
Arceus is the strongest Normal Type Pokemon with the highest Base Stat total of any Pokemon in their base forms. It was 120 Base Stats for each of its stats for a total of 720. It is able to change its typing when holding different Plates but its default form is Normal.
Slaking is the strongest non Legendary Normal Type Pokemon with a 670 Base Stat Total. It's held back by its Truant ability and can have its ability removed by various means in battle.
Mega Lopunny is the strongest Normal Mega evolution with a 580 Base Stat Total.
Fighting
Zamazenta is the strongest Fighting Type Pokemon with a 670 Base Stat Total. Its Crowned Shield form changes its stats and raises its Base Stat Total to 720. It lowers its Speed Base Stat by 10 and raises its Defense and Special Defense stat by 30 each while in this form.
Kommo-o is the strongest non Legendary Fighting Type Pokemon with a 600 Base Stat Total. The Pokemon is considered a pseudo legendary by fans of the series. Marshadow has the same number and is considered a Mythical Pokemon.
Mega Mewtwo X is the strongest Mega Fighting Type Pokemon with a 780 Base Stat Total. It is tied for the second highest Base Stat Total between Rayquaza and Mega Mewtwo Y.
Fire
Primal Groudon is the strongest Fire Type Pokemon with a 770 Base Stat Total. Its ability blocks out Water moves, effectively removing its 4x weakness.
Arcanine is the strongest non Legendary Fire Type Pokemon with a 555 Base Stat Total.
Mega Charizard X and Mega Charizard Y are the strongest Fire Type Mega Pokemon with a 634 Base Stat Total.
Related: Best Fire Type Pokémon in Pokémon Go
Water
Primal Kyogre is the strongest Water Type Pokemon with a 770 Base Stat Total.
Wishiwashi in its School Form is the strongest non Legendary Water Type Pokemon with a 620 Base Stat Total. This is reduced to 175 when in its Solo Form.
Mega Gyarados is the strongest Water Type Mega Pokemon with a 640 Base Stat Total. Although not a Mega, Ash Greninja is tied with this total.
Grass
Celebi, Shaymin, Shaymin Sky Forme, and Zarude are tied for the strongest Grass Type Pokemon with a 600 Base Stat Total. Celebi, Zarude, and Shaymin Sky Forme both have 4x weaknesses, giving Shaymin the upper hand in survivability.
Tangrowth is the strongest non Legendary Grass Type Pokemon with a 535 Base Stat Total.
Mega Sceptile is the strongest Grass Type Mega Pokemon with a 630 Base Stat Total. Mega Venusaur is only five points away from it and doesn't have a 4x weakness to Ice.
Poison
Eternamax Eternatus and Eternatus are the strongest Poison Type Pokemon with an 1125 and 690 Base Stat Total, respectively.
Naganadel is the strongest non Legendary Poison Type Pokemon with a 540 Base Stat Total. This Pokemon is an Ultra Beast, making the next highest non Ultra Beast Pokemon, Crobat with 535.
Mega Gengar is the strongest Poison Type Mega Pokemon with a 600 Base Stat Total.
Bug
Genesect, Mega Scizor, Mega Pinsir, and Mega Heracross are all tied for the strongest Bug Type Pokemon. Genesect is considered a Mythic Pokemon and the three Megas are all raised by 100 points from their base forms to reach a 600 Base Stat Total.
Pheromosa and Buzzwole are tied for the strongest non Legendary and non Mega Bug Type Pokemon at a 570 Base Stat Total. Golisopod is the next strongest that isn't an Ultra Beast with a 530 Base Stat Total.
Flying
Rayquaza, Yveltal, Lugia, and Ho-oh are the strongest Flying Type Pokemon with a 680 Base Stat Total. Rayquaza's Mega increases this to 780, tying it with the two Mewtwo Mega Evolutions.
Dragonite, both Shaymin Formes, both Landorus Formes, and Salamence are tied for the strongest non Legendary Flying Type Pokemon with a 600 Base Stat Total.
Mega Salamence is the strongest non Legendary Flying Type Pokemon with a 700 Base Stat Total.
Electric
Zekrom is the strongest Electric Type Pokemon with a 680 Base Stat Total. Zeraora follows this with 600 as a Mythical Pokemon.
Electivire is the strongest non Legendary Electric Type Pokemon with a 540 Base Stat Total.
Mega Manectric is the strongest Mega Electric Type Pokemon with a 575 Base Stat Total.
Ground
Primal Groudon is the strongest Ground Type Pokemon with a 770 Base Stat Total. Following this is Complete Forme Zygarde at 708.
Garchomp is the strongest non Legendary Ground Type Pokemon with a 600 Base Stat Total and is tied with both Landorus Formes. All three Pokemon share a 4x weakness to Ice.
Mega Garchomp is the strong Mega Ground Type Pokemon with a 700 Base Stat Total.
Rock
Both Tyranitar and Diancie are tied for the strongest Rock Type Pokemon as they share a 600 Base Stat Total. They are both capable of Mega Evolving as well, raising their stats to 700.
Related: Best Rock Type Pokémon in Pokémon Go
Ice
Black and White Kyurem are the strongest Ice Type Pokemon with a 700 Base Stat Total. Below this is Ice Rider Calyrex at 680.
Lapras and Vanilluxe are tied for the strongest non Legendary Ice Type Pokemon with a 535 Base Stat Total.
Mega Abomasnow is the strongest Mega Ice Type Pokemon with a 594 Base Stat Total.
Psychic
Ultra Necrozma is the strongest Psychic Type Pokemon with a 754 Base Stat Total. The next strongest Pokemon are Mewtwo, Hoopa Unbound, Solgaleo, Lunala, both Calyrex Rider Formes, and the two other Necrozma forms in Dawn Wings and Dusk Mane with a 680 Base Stat Total.
Metagross is the strongest non Legendary Psychic Type Pokemon with a 600 Base Stat Total.
Mega Mewtwo X and Y are the strongest Mega Psychic Type Pokemon with a 780 Base Stat Total. Mega Metagross is the strongest non Legendary Mega Psychic Type with a 700 Base Stat Total.
Ghost
Dawn Wings Necrozma, Lunala, Shadow Rider Calyrex, and both Giratina Formes are tied for the strongest Ghost Type Pokemon with a 680 Base Stat Total.
Marshadow and Dragpult are the strongest non Legendary Ghost Type Pokemon with a 600 Base Stat Total.
Mega Gengar is the strongest Mega Ghost Type Pokemon with a 600 Base Stat Total.
Dark
Yveltal and Hoopa Unbound are tied for the strongest Dark Type Pokemon with a 680 Base Stat Total.
Zarude, Hyrdreigon, and Tyranitar are tied for the strongest non Legendary Dark Type Pokemon with a 600 Base Stat Total.
Mega Tyranitar is the strongest Mega Dark Type Pokemon with a 700 Base Stat Total.
Steel
Dusk Mane Necrozma is the strongest Steel Type Pokemon with a 754 Base Stat Total. Crowned Sword Zacian and Crowned Shield Zamazenta are the next strongest Steel Type Pokemon with a 720 Base Stat Total. This is 50 points higher than their Hero of Many Battles form.
Metagross, Melmetal, Magearna, and Genesect are tied for the strongest non Legendary Steel Type Pokemon with a 600 Base Stat Total.
Mega Metagross is the strongest Mega Steel Type Pokemon with a 700 Base Stat Total.
Dragon
Eternamax Eternatus and Eternatus are the strongest Dragon Type Pokemon with an 1125 and 690 Base Stat Total, respectively.
Dragapult, Salamence, Dragonite, Garchomp, Hydreigon. Goodra, and Kommo-o are all tied for the highest non Legendary Dragon Type Pokemon with a Base Stat Total of 600.
Mega Salamence and Mega Garchomp are the strongest non Legendary Mega Dragon Type Pokemon with a 700 Base Stat Total. Mega Rayquaza is the strongest Mega Dragon Type Pokemon at a 780 Base Stat Total.
Fairy
Crowned Sword Zacian is the strongest Fairy Type Pokemon with a 720 Base Stat Total. The second highest is Xerneas and Solgaleo at 680.
Diancie and Magearna are the strongest Mythical Fairy Type Pokemon with a 600 Base Stat Total. Florges follows this as a non Mythical Pokemon at 552.
Mega Diancie is the strongest Mega Fairy Type Pokemon with a 700 Base Stat Total. Mega Gardevoir follows this as the strongest non Mythical Mega with a 618 Base Stat Total.
Although this shows the strongest Pokemon by Base Stats, there are other factors that can determine a Pokemon's effectiveness in battle. Strategy and training can make even a level one Rattata take out the strongest of Legendaries.
For more guides on Pokemon, check out Which Pokémon should I get: Brilliant Diamond or Shining Pearl? on Pro Game Guides.
