>Kanto
Nidoking
Available before the 1st Gym, can fully evolve before the 2nd, and learns damn near every move in the game via TMs.

>Johto
Quagsire
Available before the 1st Gym in HGSS, evolves super early and bodies everything the game throws at it. Also learns some of the strongest moves in the game, and is one of the few Pokémon that learn Earthquake by level up in GSC.

>Hoenn
Crobat
Can catch right before the Fighting Gym. Very dependable throughout the rest of the game, and makes needing a Fly slave unnecessary.

>Sinnoh
Garchomp
You get access to the strongest pseudo in the game right after the 2nd Gym. I think that says enough.

>Unova
Stoutland
Can catch right from the beginning. Learns great coverage and has decent stats to back it up. Like Crobat, it's just very dependable.

>Kalos
Talonflame
First Route 1 bird to not be Normal/Flying. Has good stats and movepool, and helps with hatching eggs. Also takes the spot of a Fly slave.

>Alola
Lycanroc
Available on the first island, then pretty much steamrolls the rest of the game. Accelerock lets it hit hard and fast.

>Galar
Corviknight
The single best Route 1 bird in existence, with a typing that walls every damn thing in the game. This one doesn't need much explaining.