Location: Johto

Method:
This Electric-type legendary is an excellent choice for battle. With only one weakness and a great Special Attack stat, Raikou is ready to inflict serious damage with moves like Crunch and Thunder. For an interesting combo, try using Rain Dance to summon a storm on the first turn, then hit with never-missing Thunders on subsequent turns.

Raikou Level 40

OT: Yours ID: Yours Hold Item: None

Leer Thundershock Roar Quick Attack

Location: Johto

Method:
The Fire-type Entei can be tricky to use. Although he has incredible HP and Attack, his physical move set is rather lacking and he cannot benefit from both his titanic power and his strong STAB move Fire Blast. Despite this, Entei does get moves like Iron Tail and Strength to dish out damage, and it doesn't have abysmal Attack. Entei gets Solarbeam so use that in conjunction with Sunny Day for decent coverage.

Entei Level 40

OT: Yours ID: Yours Hold Item: None

Leer Ember Roar Fire Spin

Location: Tin Tower

Method:
An awesome bulky Water-type Pokemon with high HP and defenses. Suicune is great for whittling down tough foes with Toxic or Roaring them away for later, but with moves like Surf and Blizzard it can also function as an excellent fighter. This is the only chance to get one, but you don't have to worry about it fleeing from battle or using Roar to end the match. Make sure you catch it for what's next.

Suicune Level 40

OT: Yours ID: Yours Hold Item: None

Leer Bubblebeam Rain Dance Gust

Location: Whirl Islands

Method:
The Silver Wing is obtained from an old man in Pewter City after beating the Elite Four. Take it to the top right of the Whirl Islands and proceed on your way. Lugia has among the highest defenses in the game but can also function as an attacker due to its above average Attack and Special Attack stats. Try a Heavy Ball on it for an easier catch.

Lugia Level 60

OT: Yours ID: Yours Hold Item: None

Recover Hydro Pump Rain Dance Gust

Location: Tin Tower

Method:
The RainbowWing is obtained from Eusine after catching the three dogs and showing him in Celadon City. He will bring you to Tin Tower and allow you up the previously blocked stairway. Ho-oh always carries the Sacred Ash, which fully heals your entire party. Use it if you need it against Trainer Red.

Ho-Oh Level 60

OT: Yours ID: Yours Hold Item: Sacred Ash

Recover Fire Blast Sunny Day Gust

Location: Ilex Forest

Method:
To acquire a GS Ball you have to connect the game to the mobile phone with the Pokemon Mobile Phone System GB. This cannot be done in the US. You must have all 16 badges, beat a minigame, and complete a quiz to have the attendent at the Pokemon Communication Center in Goldenrod City give you a GS Ball.
In the Virtual Console release, you will receive the GS Ball at the Goldenrod City Pokémon Center after you have defeated the Elite Four, without needing to go through Kanto.
Once you have the GS Ball, take it to Kurt to have him inspect it. After the turn to the next day he'll tell you about a strange presence in the ball before dashing outside. Follow him and he'll say Ilex Forest is uneasy and that you should check it out with the GS Ball.
The trees will be rustling in the forest. Take the GS Ball to the shrine. When asked if you want to put the GS Ball in the shrine, say yes and Celebi will arrive to challenge you. Celebi holds no special items. Grass/Psychic is weak to Fire-, Flying-, Ghost-, Dark-, Ice-, and Poison-types and has a double weakness to Bug-types but offers good STAB and learns powerful moves like Giga Drain, Psychic, and Solarbeam. Because you need all 16 badges to fight it but is faced at a low level of 30, it would be wise to bring something that can inflict status before it uses Safeguard but not OHKO it.

Celebi Level 30

OT: Yours ID: Yours Hold Item: None

Heal Bell Safeguard Ancientpower Future Sight