10POWERFUL: MEWTWO
Canonically one of the strongest Pokémon in existence, it makes sense that Mewtwo would also be one of the most viable Pokémon in the games. Its stats allow for it to throw out a lot of damage, its move pool lets it cover a lot of types, and its two Mega Evolutions give it the versatility that it didn't need in the first place. Game Freak set out to create the world's strongest Pokémon and they definitely succeeded with Mewtwo.
9WEAK: PHIONE
Phione is often a legendary Pokémon that people forget about. That's partially because it's kind of a baby version of Manaphy, but it's also because its stats leave a lot to be desired. Manaphy is better in every way, being a Pokémon that has a lot more potential. It has the same number for each of its stats, but that number is so low that Phione is better to be never used. It can't learn any seriously useful moves either, being almost worth nothing.
8POWERFUL: RAYQUAZA
Rayquaza was a monster before Pokémon Omega Ruby and Alpha Sapphire, and it became a much bigger threat after that. Rayquaza and Salamence remain the strongest Pokémon from Gen III, and both of them got Mega Evolutions. Mega Rayquaza can counter the permanent weather changes from Primal Groudon and Primal Kyogre. It also hits like a truck, dealing a lot of damage, having great coverage, and not being weak to many attacks. Rayquaza pops up a lot in competitive play to either counter other Pokémon or to carry an entire team.
7WEAK: REGIGIGAS
Regirock, Regice, and Registeel were part of the legendary trio in Gen III. However, their existence merely pointed to the awakening of an even larger golem, Regigigas in Gen IV. Despite us waiting an entire generation to see the lord of the golem trio, it wasn't worth the wait. It has the ability Slow Start, which cuts its attack and speed stat in half for five turns when it switches into combat. Its stats would've been otherwise decent, but the ability alone makes it a serious disappointment.
6POWERFUL: GROUDON
Groudon was a powerful Pokémon when it was first introduced. It had some dangerous ground-type attacks and the stats to back it up. However, it was a Pokémon that used the sun in battle but didn't get STAB on its fire moves because it wasn't a fire type. That changed in Omega Ruby when Groudon was given a Primal Reversion to become Primal Groudon. It gained the fire typing and could summon permanent sunlight with its ability. Groudon suddenly became a much bigger threat.
5WEAK: REGI TRIO
Regirock, Regice, and Registeel had cool designs and it took a while to find them in the games. However, the adventure necessary to obtain them wasn't worth it because of how poor they were in battle. None of them had great stats or great move pools. Their single typings only shrunk their move pools. They each had a lot of weaknesses to common types. Furthermore, any potential they had was wasted by Pokémon that did everything they could do but much better.
4POWERFUL: KYOGRE
Kyogre was already powerful before it gained a Primal Reversion due to it getting STAB on its rain-enhanced water moves, but it became a larger threat after Alpha Sapphire's release. What makes Primal Kyogre such a beast is that its stats are increased to give it some serious attack power on top of keeping its bulk. This makes Kyogre an unpredictable asset to any team. There's no telling what someone will use a Primal Kyogre for, because it excels at just about anything.
3WEAK: VIRIZION
Virizion is the weakest of the Swords of Justice from Gen V, and none of them are particularly that great to begin with. The reason Virizion is at the bottom is because it takes a lot of damage from physical attacks but doesn't have the defense to take a hit. What's worse is its grass and fighting type, making it four times weak against flying attacks. Considering a lot of popular Pokémon are flying types with much greater speed, Virizion has no chance. Even its Z-move can't save it.
2POWERFUL: ARCEUS
It makes sense that the god of Pokémon would climb high up the tier lists. Arceus has everything you'd want in a Pokémon: great stats, a great moveset, and flexibility. With a simple item, Arceus can change to any type in the game. This type also changes the type of its signature move, Judgment, guaranteeing a STAB boost at all times. Even without a type change item, Arceus is a sheer monster that many players don't know how to handle.
1WEAK: MESPRIT
Mesprit is one of the legendary Pokémon trio from Gen IV, and it's every bit as underwhelming as you might expect. The big problem with Mesprit is that its stats are working against it at from all angles. There is just no redeeming quality to Mesprit. While a single Psychic type doesn't give it a lot of weaknesses, it doesn't have the attack or defense to make good use of it. When Pokémon like Mewtwo and Deoxys already exist, there's no reason to choose Mesprit over them.
0POWERFUL: YVELTAL
Yveltal is a deity of chaos and was one of the legendary Pokémon introduced in Gen VI with Pokémon X and Y. Yveltal lives up to its moniker by having some deadly stats. This dark and flying type wyvern has the stats to work in just about any position on a team. Yveltal has a wide move pool and has the power to back it up. It was also given a signature move in the form of Oblivion Wing, a powerful flying attack that recovers health to the user.