I love Pokémon and have been watching the anime/cartoon ever since I was a little child! Pokémon is still popular!
Yes, one of the most powerful Pokémon in the Pokémon universe is from the first generation. Back in the first season, psychic types were just ridiculously strong. Mewtwo, being one of the first legendary Pokémon, still hits incredibly hard with its signature move, psystrike. When mega evolution was first introduced in XY, Mewtwo was given two mega evolutions instead of one (the other Pokémon that was given this special treatment was everyone's favorite Charizard).
Lugia is regarded as the "guardian of the seas". This Pokémon is also the leader of the legendary birds, Articuno, Moltres, and Zapdos. If the legendary birds are fighting each other, Lugia is the only Pokémon that can stop them. In addition to its raw power, Lugia is very intelligent. Because of its immense power, it stays out of sight and sleep at the bottom of the ocean. When Lugia is in action, it can control weather and use its signature move, aeroblast.
Ho-OH, the counterpart of Lugia, is the "guardian of the skies". This Pokémon is also the leader of the legendary beasts, Entei, Raikou, and Suicune. Ho-OH looks like a phoenix and has incredible fire power to back up its fierce looks. With scared fire as its signature move, it's able to demolish most Pokémon with ease. Most legendary Pokémon will not appear in the anime, Ho-OH is one of the exceptions. Appearing in the very first Pokémon episode where Ash saw it flying across the sky. After Ash completed the Silver Conference, this legendary bird appeared again and flew toward the Hoenn region. It is said that Ho-OH will search for pure hearted trainers; that means Ash Ketchum is a very special trainer!
Rayquaza is one of the most powerful legendary Pokémon. This Pokémon is so powerful both in the anime and the Pokémon game. And more surprisingly, Rayquaza has a mega evolution to make this already powerful Pokémon even more amazing. Rayquaza leads the weather trio which also includes Kyogre and Groudon. With its signature move, dragon ascent, it is able to quell the fighting between Kyogre and Groudon. What's even more surprising? Rayquaza can mega evolve by itself. With this unique ability, it's able to hold an item to increase its already overwhelming power. A shiny mega Rayquaza made its appearance in the Hoopa movie. The black shiny mega Rayquaza looks even more fierce than the normal green colored one.
This Pokémon is one of the most powerful in all of Pokémon universe. It is the trio master of both the lake guardians and creation trio. It has the highest stats of any Pokémon prior to the introduction of mega evolutions. Other than incredibly high stats, Arceus is also able to change type depending on the plate it's holding or specific type of Z-crystals. Many different types of plates are available for Arceus, that means this Pokémon can change into any type. How terrifying! Imagine in the middle of a battle, this god-like Pokémon can easily demolish your Pokémon by changing itself into a type that you are weak against. With its signature move, judgement, this Pokémon will go on a rampage if it's betrayed by friends. But if you treat this Pokémon with kindness, it will treat you with compassion and kindness in return.
This ghost and dragon type legendary Pokémon is so amazingly cool. It will change from its origin form to altered form when it comes to the real world from its distorted home world. This change in form is due to the change of gravity. But both forms look very good and are equally powerful. Even though it's said to be very aggressive, it protected the real world from Cyrus. This legendary Pokémon is on the top of the list for ghost type trainers!
Kyurem is already a strong legendary Pokémon. However, it's able to "merge" with either Reshiram or Zekrom to form white Kyurem or black Kyurem. With already high base stats at 660, merging with other legendary Pokémon will boost its base stats to 700. Watch Kyurem in action in one of the Pokémon Generation episodes. Ice type legendary Pokémon is very rare. Kyurem is the perfect choice for ice type trainers!
This legendary Pokémon has different forms, core, 10%, 50%, and 100%. Zygarde looks like transformer when it's in the 100% form. This legendary Pokémon's core form was actually introduced in the Pokémon anime. Even though it debuted as a tiny little green Pokémon that looks completely useless and nicknamed by one of the main characters as "squishy", it's 100% form's power exceeds both Xerneas and Yveltal. It is said that when the ecosystem of Kalos falls into disarray, this Pokémon will release its secret power. That's what we need in the real world to heal the earth against pollution, global warming, and the destruction of habitats.
Zygarde and its different forms
This is the newest legendary Pokémon in Sun and Moon. Like Kyurem, it can fuse with other legendary Pokémon to make itself even more powerful. In addition to its two signature moves (prismatic laser and photon geyser), it can even use its own Z-move. But, wait, there is more. When fused with the other two legendary Pokémon (Solgaleo and Lunala), Necrozma can even learn their signature moves and Z-moves as well. So, it's the only legendary to have multiple exclusive moves.