Since the franchise's beginning, Pokémon has always released two versions of each main game with only a few differences between them. The first and most noticeable is the box-art Legendary. Sometimes, the developers will include version-exclusive Gym Leaders. However, the third most obvious difference is the individual Pokémon exclusive to each version.
Remakes Brilliant Diamond and Shining Pearl are no exception, so they too have exclusive Pokémon from previous generations. Brilliant Diamond has a slew of unique Pokémon that could make all the difference in a playthrough, and fans might feel inclined to choose it over Shining Pearl after seeing how powerful they can be.
Honorable Mention: Ho-Oh
Ho-Oh is one of the franchise's most emblematic creatures and one of the Legendary Pokémon with the best lore. It is a dual Fire/Flying-type introduced in Generation II, closely associated with another Legendary. Lugia. Ho-Oh has a base stat total of 680, boasting above-average everything with particular emphasis on Special Defense.
Even though Ho-Oh is only obtainable after obtaining the National Pokédex and catching the three Legendary Beasts, it's still worth the effort. Sinnoh is infamous for its lack of strong Fire-types, so having a Pokémon like Ho-Oh could make a major difference.
Honorable Mention: Legendary Beasts
Speaking of the Legendary Beasts, these massive dogs appear in Ramanas Park after the player catches the Legendary Golems. Entei, Raikou, and Suicune are three of the most majestic Pokémon around, and any player would be lucky enough to have them on their team.
By being Legendary Pokémon, the Beasts have impressive stats, with Entei favoring HP and Physical Attack, Suicune leaning towards Physical and Special Defense, and Raikou being a Special Attacker with excellent Speed. There are no downsides to using the Beasts, which explains why they are available after the player enters the Hall of Fame.
Scizor
Steel might be the best type in Pokémon thanks to its numerous resistances and its Super Effectiveness against Fairy. Thus, Steel-types become a Trainer's best friends, and there's no better friend than Scizor, the dual Bug/Steel-type introduced in Generation II. It can become part of a players' team before the sixth Gym badge. Its pre-evolution, Scyther, roams in the Grassland Caves at the Ground Underground, and the Metal Coat needed to evolve it is obtainable at Iron Island.
Scyther evolves into Scizor via Trade, which is bothersome. However, it's more than worth the effort. Scizor has only one weakness and a base stat total of 500 that ranks it as one of the most powerful non-Legendary Pokémon in the games.
Tyranitar
Every Pseudo-Legendary Pokémon is special in its own right, and they all place highly in any non-Legendary ranking. Sinnoh has its own very famous Pseudo, Grachomp, but Brilliant Diamond offers players another option in the form of Tyranitar.
This kaiju-like Rock/Dark-type is obtainable after the player enters the Hall of Fame and receives the National Pokédex. However, it's worth the wait: Tyranitar boasts a massive 134 Physical Attack that makes it a living weapon. It might be slow, but its excellent defenses guarantee it'll tank almost any attack, retaliating with devastating fury.
Electivire
Like Steel, Electric is also one of the best types in the franchise. It only has one weakness, Ground, and is Super Effective against the absurdly common Water-type. Iconic Generation I Pokémon Electabuzz was already a decent ally to have, but Gen IV turned it into a force to be reckoned with by giving it an evolution, Electivire.
Thanks to its gargantuan 540 base stat total, Electivire is a dangerous threat. As a pure Electric-type, it only has one weakness, and it resists three more types, including the menacing Steel. Electivire also makes the most out of moves like Thunder Punch and Wild Charge, thanks to its massive 123 Physical Attack, making it one of Sinnoh's strongest Electric-types.
Arcanine
Sinnoh is infamous for lacking Fire-types aside from Rapidash and Infernape. Things get better once the National Dex becomes available, introducing a parade of Fire-types that finally balance things out.
Growlithe and its evolution, Arcanine, are among the Pokémon that even non-fans of the franchise know. However, Arcanine is also immensely powerful, with a base stat total of 555 that puts most other Pokémon to shame. Thanks to its excellent Physical and Special Attack and above-average Speed, Arcanine can OHKO most opponents, making it one of the most valuable Fire-types in the franchise.
Skuntank
Poison and Dark, the two "evilest" types in Pokémon, mix to create Skuntank, the "evilest" Pokémon in Sinnoh. This purple menace is Commander Jupiter's Ace and can give considerable trouble to players, mainly because it only has one weakness: Ground.
In the stat department, Skuntank is no pushover either: it boasts an above-average 479 total, favoring HP, Physical Attack, and Speed. Skuntank is also considerably better than its Shining Pearl counterpart, Purugly, making it even more appealing for Brilliant Diamond players.
Honchkrow
The Dark-type is a player's best ally in the Sinnoh region. It is Super Effective against the most challenging Gym Leader in Pokémon Brilliant Diamond, Fantina, and will be a life savior against Lucian, the toughest member of the Elite Four. For its part, Flying will also come in handy against Gym Leaders Gardenia and Maylene. As such, any dual-type Dark/Flying Pokémon will be the MVP of the game.
Fortunately, BD has one such Pokémon, Honchkrow. This Dark/Flying crime boss has an excellent 505 base stat total that favors Physical and Special Attack, making it an offensive ally that any player would be lucky to have.
Gliscor
Generation IV introduced new evolutions to several single-stage Pokémon. However, few are as powerful or useful as Gliscor, Gligar's evolution. Ground and Flying perfectly complement each other, leaving Gliscor with only two weaknesses, plus two immunities. Its base stat total of 510 Physical Attack and Defense, while also paying extra attention to Speed.
The result is an excellent Pokémon that can make all the difference in a playthrough. The only downside is that the Razor Fang, the item necessary to evolve Gligar, is unavailable until after beating the Champion. However, there are ways around this, and Gliscor is certainly worth the effort.
Dialga
Dialga is arguably the best Legendary Pokémon in Generation IV. This dual Steel/Dragon-type has one of the coolest type combinations in the franchise and a design that makes most other Legendaries look like run-of-the-mill Route 1 Pokémon. On top of that, Dialga is a literal god, controlling the flow of time in the Pokémon universe.
If all those assets weren't enough, Dialga is also absurdly powerful, with a base stat total of 680 that ensures every stat is above the 100s, except for Speed. Dialga is undeniably cooler than its Shining Pearl counterpart, Palkia, and it's no surprise that most players consider it one of the all-time best Legendary Pokémon.