Here is a list, up to date as of S/V, showing the Pokemon with the highest in each base stat, excluding megas and unobtainable forms:
- HP: Blissey at 255
- Attack: Kartana at 181
- Defense: Shuckle at 230
- Sp. Attack: Tied between Primal Kyogre and Attack Form Deoxys
- Sp. Defense: Shuckle at 230
- Speed: Regieleki at 200
- Total: Tied between Primal Kyogre and Primal Groudon at 770
Note: This answer technically states the base stats of Pokémon, but if you all give them the same item, or nature, or really anything that can raise stats, it’s all the same rankings, because they are all raised by the same percentage.
Source: My answer on another similar question
Hope I helped!
P.S. I know for a fact that this is a copy of at least of at least one other question, but I’ll leave it up to the mods to decide which to keep up. Although, I do know that it is not a copy of this one because that one asks for non-legendary Pokémon.