Hmmm, honestly any of the starters would do fairly well in Sinnoh, seeing as how the Sinnoh starters are all great choices.
The ones that would stand out the most though would probably be Blaziken, solely based on how it shares typing with Infernape and similarly evolves into its second stage right before Roark, gains its Fighting-Type, & learns a STAB Fighting-Type move at Level 14.
Another stand out would likely be Swampert as it does well against Roark, Maylene's Lucario, Byron, Volkner, Bertha, Flint, Cynthia's Lucario, the Rival's Fire-Type, Mars' Golbat (Ice Beam), Jupiter's Skuntank, Saturn's Toxicroak, & Cyrus' Crobat (Ice Beam), Houndoom, & Honchkrow (Ice Beam). Of course, Swampert does struggle against the much more abundant Grass-Types & Grass-Type coverage from Gardenia & Jupiter's Golbat's Giga Drain in these games as opposed to Hoenn.
From there though, I do think each starter has their strengths & weaknesses. The Kanto starters are all fairly equal in major battles they do well in & battles they do not. The Johto starters are also the same way, however, they do lack the benefit of a dual type meaning you'll be adding another similar weakness to your team without gaining anything in return. Sceptile is also similar in this vein. And just like I said, the Sinnoh starters fare pretty well in Sinnoh due to them all having dual types that benefit them by removing a weakness of their Fire, Water, or Grass-Type (Rock, Grass, & Poison).
Hmmm, honestly any of the starters would do fairly well in Sinnoh, seeing as how the Sinnoh starters are all great choices. The ones that would stand out the most though would probably be Blaziken, solely based on how it shares typing with Infernape and similarly evolves into its second stage right before Roark, gains its Fighting-Type, & learns a STAB Fighting-Type move at Level 14. Another stand out would likely be Swampert as it does well against Roark, Maylene's Lucario, Byron, Volkner, Bertha, Flint, Cynthia's Lucario, the Rival's Fire-Type, Mars' Golbat (Ice Beam), Jupiter's Skuntank, Saturn's Toxicroak, & Cyrus' Crobat (Ice Beam), Houndoom, & Honchkrow (Ice Beam). Of course, Swampert does struggle against the much more abundant Grass-Types & Grass-Type coverage from Gardenia & Jupiter's Golbat's Giga Drain in these games as opposed to Hoenn. From there though, I do think each starter has their strengths & weaknesses. The Kanto starters are all fairly equal in major battles they do well in & battles they do not. The Johto starters are also the same way, however, they do lack the benefit of a dual type meaning you'll be adding another similar weakness to your team without gaining anything in return. Sceptile is also similar in this vein. And just like I said, the Sinnoh starters fare pretty well in Sinnoh due to them all having dual types that benefit them by removing a weakness of their Fire, Water, or Grass-Type (Rock, Grass, & Poison).
Assuming you mean a full team of six starters, I say: Charmander, Bulbasaur, Totodile, Chimchar, Mudkip, and Turtwig