I have just finished a Gold walkthrough with a team of Meganium, Raticate, Scyther, Dewgong, and Rhydon (in-game trade). I wanted to add a Fire-type Pokemon (Ho-Oh) but it would seem redundant with Scyther's, Dewgong's, and Rhydon's STABs. Since Chikorita has a write-up in the OP, I'd like to contribute some write-ups for the other Pokémon:
Name: Rattata
Availability: Early Game, it is available at the first routes you encounter (Routes 30, 31, 32, 33, and 34). Rattata is much more common at night.
Stats: Good Attack and Speed stats for in-game purposes. It is rather frail though.
Typing: Normal STAB is useful in-game for neutral damage to majority of trainers.
Movepool: It learns Hyper Fang at Level 13, which is incredibly early for an 80 BP STAB. It also learns Quick Attack to pick off weakened threats and missed KO's. By virtue of being a Normal-type, Raticate learns a wide selection of TM including Shadow Ball, Dig, Iron Tail, and Icy Wind.
Major Battles: Rattata fares well against Falkner (useful if you chose Chikorita) and Bugsy. Mud Slap or Focus Energy gives it the tools to beat Whitney's Miltank, either by lowering Rollout's accuracy or getting a lucky critical hit. Raticate also handily beats Morty as well if you give it Dig; you wall its Ghost Attacks though Raticate has to switch out if it gets hit by Curse. Late-game, it can OHKO off some frail boss Pokémon such as Will's Jynx, Koga's Ariados and Crobat, and Karen's Murkrow.
Additional Comments: Rattata shines best early/mid-game and peaks once it evolves into Raticate. STAB Hyper Fang and Ground coverage allows it to solo Team Rocket Grunts by itself. Mono-Normal STAB doesn't give it a huge advantage on late-game bosses though its stats and Hyper Fang allow it to clean route trainers efficiently.
Name: Scyther (No trade)
Availability: Early but rare, available at the Bug Catching Contest during Tuesdays, Thursdays, and Saturdays.
Stats: Its Attack and Speed stats are perfect for in-game runs. Its bulk is also decent as well.
Typing: Offensively, Bug and Flying STABs are good and fare well against a lot of route trainers, Elite Four, and Kanto gym leaders. Defensively, Ground immunity and double Grass and Fighting resistances are its best assets. Double weakness to Rock sucks though as Fighting-types can surprise it with a Rock Slide which will instantly KO Scyther.
Movepool: Early-game, Scyther will have to rely on Quick Attack and Fury Cutter (TM) to deal damage. Its best STAB, Wing Attack, comes at level 30. It also learns useful moves late-game in Slash and Swords Dance. Besides Fury Cutter, the only useful TMS it learns are Steel Wing and Return.
Major Battles: Scyther's double resistance to Fighting gives it an advantage over Chuck, though it is unlikely that it knows Wing Attack by that stage so Poliwrath may still beat it with Surf. Elite Four is where Scyther shines the best though as it can beat a huge chunk of their teams with Swords Dance + Wing Attack and Slash. In Kanto, Scyther sweeps Erika handily while also performing fine against Misty, and Sabrina. Against Blue, it can take on Alakazam, Exeggutor, and Gyarados. It is walled by Brock though, and Lt. Surge's Electric Attacks will overwhelm Scyther.
Additional Comments: Despite its high stats, Scyther struggles early game bosses to to a poor movepool at that stage. Its high Attack means that it can still keep up with the team by beating route trainers and wild Pokémon. Scyther picks up once it learns Wing Attack, and peaks late-game once it learns Swords Dance at level 42.
Name: Seel
Availability: Late-game. Is is available once you are able to use the HM Whirlpool field move after beating Pryce. It requires a bit of detour at Whirl Islands between Olivine City and Cianwood City and is available at Level 25.
Stats: Dewgong's stats are average overall, with below-average offenses and above-average defenses.
Typing: Dewgong's STABs are fantastic in-game. Water and Ice STABs perform well against a lot of late-game bosses, Kanto Gym Leaders, and Cave Pokémon. Having STAB on its Water and Ice moves also compensate for Dewgong's below average Special Attack. Unlike other Water Pokémon, don't expect it to wall Fire-type moves, as its Ice-typing makes it neutral to them.
Movepool: Seel's biggest advantage over other Water-types is that it learns Ice Beam by level up, as Ice Beam is not a TM in Pokémon Gold and Silver. It also learns useful STAB HMs in Surf and Waterfall, so it is not at all reliant on consumable TMs.
Major Battles: With STAB Ice Beam or Aurora Beam, Dewgong sweeps Clair's Dragonairs while also performing well against Kingdra. It also performs well against Lance Surf and Ice Beam KO's 5/6 of his Pokemon. Its low Speed though means that it has to use some Hyper Potions and it also has to watch out for the Dragonite with Thunder. Dewgong's Kanto performance is also fine: it sweeps Erika, Brock, and Blaine while also performing well against Janine and Blue. Dewgong can also handle Blue's Pidgeot, Rhydon, Exeggutor, and Arcanine.
Additional Comments: Dewgong learns Ice Beam at Level 43 while Seel learns it at Level 37. It is advisable to hold off Seel's evolution for 3 levels so that Dewgong will have the powerful Ice Beam once it faces Clair.
Name: Rhydon (in-game trade)
Availability: Late-game in an in-game trade at Blackthorn City. The girl requires a female Dragonair. To get a female Dragonair you either:
1) purchase a Dratini early in the game (quite expensive) and put it in the daycare to level up then evolve it into Dragonair once it reaches Level 30 or
2) you catch a Level 20 Dratini in the Dragon's Den then grind it up (or use Rare Candies) until it evolves into a Dragonair at Level 30.
Stats: High HP, Attack, and Defense stats make Rhydon a physical powerhouse though its low Speed and special stats hold it back.
Typing: Ground/Rock is a good offensive combo and goes unresisted this generation, though it lacks a consistent Rock STAB. The typing also allows it to wall Electric-, Flying-, and Normal-types. Unfortunately, the same typing also gives it double weaknesses against Water- and Grass-types.
Movepool: Rhydon needs to use the Earthquake TM to obtain its optimal performance. Rhydon is an incredibly powerful user of Earthquake so the sacrifice is more than worth it. Otherwise, it can still use Dig. It's only Rock STAB in this game is Rollout. It can also use powerful Normal moves in Return and Strength (for field utility).
Major Battles: Rhydon can beat Koga though it certainly has to avoid Ariados' Giga Drain. It can beat Lance's Aerodactyl, Charizard, and the Thunder and Fire Blast Dragonites, though it loses to his two other Pokémon. Rhydon's Kanto matchups are great as it sweeps Lt. Surge, Blaine, and Janine while also doing good damage against Sabrina's physically frail Pokémon. Rhydon notably walls and beats 3 of Red's Pokemon: Pikachu, Snorlax, and Charizard.
Additional Comments: Rhydon's in-game trade status allows it to level up faster than its teammates, so catching up isn't a hard problem for the Drill Pokemon. It's powerful but slow so it might not able to successfully run against faster wild Pokémon. Using a Quick Claw can circumvent its Speed problem in battle. It's also good to pack X-Accuracy in case Rhydon wants to sweep with Rollout.