While Brilliant Diamond and Shining Pearl are on the way, the original Sinnoh Pokémon games always had a popular rotation of creatures that players chose for their team rosters. With this region getting a modernized facelift with the aforementioned remakes, several of these picks will likely retain and increase their popularity, though, new species may also work their way in for a new generation of players given the improved creative freedom.
In certain cases for the 2007 games, this amounted to the fourth generation having a plethora of fan-favorite species with inventive character designs to choose from. Meanwhile, other picks were from restricted choices in the given Pokémon's typing and overworld traversal.
Infernape
The Fire-type starters were always among the most popular beginning with generation one, and Infernape establishes itself as one of the strongest beginners picks for Diamond and Pearl. Infernape repeats the Fire/Fighting dual typing from gen three's Blaziken but remains one of the heaviest hitters in Sinnoh.
It also helped that it can learn some of the most powerful Fire-type Pokémon moves available, like Flare Blitz, and its physical attack stats help bolster the Fighting-type attacks it can also learn like Close Combat. However, it's also one of the most practical choices since, before Platinum was released in 2009, Diamond and Pearl were severely limited when it came to Fire-type species in the wild.
Luxray
This Electric-type Pokémon is a great blend of in-game power, sleek design, and convenience for most players of the classic DS games. Luxray's first form in its evolutionary line, Shinx, can be found in some of Diamond and Pearl's earliest routes, making it an easy early pickup for players looking to fill out a slot in their team.
Electric-type Pokémon moves are useful in the Sinnoh region, including against the fourth gym leader Crasher Wake who specializes in Water-type species. The male lion/lynx/sphinx inspiration also makes for a slick creature design that's hard to pass up, even if only knowing what Shinx's aesthetics are.
Staraptor
Similar to Luxray, Staraptor is an easy pick for the final team roster of a Diamond or Pearl playthrough thanks to aesthetics and convenience. The first form in this evolutionary line, Starly, can be found in the wild almost immediately since it's one of the "regional bird Pokémon" of the game series that's become a tradition or staple of the franchise.
Especially at the time of the mainline games, having a Flying-type was still a requirement for fast travel across the region, but Staraptor has the power to make that up in battle. It could even learn powerhouse attacks like the Fighting-type move Close Combat despite being a dual Normal/Flying-type bird species.
Floatzel
Like needing to have a Pokémon with access to Fly for fast travel, having a Water-type on the team was practically necessary to surf across Diamond and Pearl's water routes. Including Buizel, which relatively soon after in leveling evolves into Floatzel, there are four quick pickups in the wild to form the majority of the player's team in the early portion of the games.
What makes getting a Floatzel so convenient is that it's an early-route Water-type that can be found in the wild grass just after leaving Floaroma Town. While having to sacrifice a team member's move slot for an HM, Surf is still a strong Water-type Pokémon move to have regardless of overworld traversal requirements.
Bibarel
In the original games -- and until Pokémon Sun and Moon -- the Hidden Machine (HM) moves were one of the biggest nuisances of the mainline series. They were moves that couldn't be easily deleted and were required in many cases to progress through the story. Oftentimes, the HM moves weren't particularly useful in battle either. Doing away with them was one of the most important quality-of-life changes in the Pokémon franchise but until then, D/P players often opted for sacrificing a team slot for an "HM mule."
That's where Bibarel comes in, being a jack-of-all-trades Normal-type. Bidoof is also found almost immediately and by the time it evolves into Bibarel, it can learn six of the HMs in these games. Rationing out HMs across the team, though, is another option so players wouldn't have to essentially operate with a team of only "5+1" with the "mule."
Rapidash
Though Infernape already fills the slot of the Fire-type at the start of the game upon picking Chimchar, Rapidash was the common alternative if players chose Turtwig or Piplup as their starter instead. Diamond and Pearl's restricted amount of Fire-types made it to where Rapidash was the only alternative.
The fourth generation of Sinnoh did introduce the likes of Magmortar, but they were locked behind the post-game portion of the games. Platinum expanded the regional Pokédex with Fire-types from past regions but before that Infernape or Rapidash were the only realistic choices. It was a shame, especially since Rapidash is on the weaker side of that typing pool and one of the Elite Four members was a Fire specialist, but only had two on his team of five.
Lucario
Despite only being able to get one in-game (hatching a Riolu egg), having to grind friendship points to evolve into one, and getting one late in the games' story, Lucario is among the most popular picks for a Sinnoh team. Lucario is also one of the most popular Pokémon in general and has the battle prowess to be viable.
The dual Steel/Fighting-type creature was one of the major "mascots" for the Sinnoh region in the lead-up to Diamond and Pearl's releases thanks to its creative, Anubis-inspired design. Combining one of the strongest types in Steel with Fighting and its wide movepool also made it quite versatile. Its high Physical and Special Attack stats made it the best of both worlds on the offense.
Garchomp
In terms of raw power, Garchomp was a go-to Pokémon unless players didn't mind having one of the Legendary creatures on their team. It is the pseudo-Legendary of Sinnoh given its daunting stats, and also since the Dragon-type species were still not as opposed like when the Fairy-type was introduced.
Pseudo-Legendaries are usually hard to come by, but one of the big perks of Garchomp was that it was surprisingly fast to catch Gible -- its first form. While it takes time to get it to fully evolve, Gabite serves as a worthy contender until that time finally comes. Its excellent land-shark design is also among Sinnoh's best and the dual Dragon/Ground typing is a force to be reckoned with.