The best fire type Pokemon (excluding Ho-Oh and Entei) in G/S is probably Magmar. The others require Fire Stones which are limited to 1 in GS AND are post-game. It also has great offensive stats and starts with Fire Punch. It is also pretty early, being in Burnt Tower. It also has a good enough offensive movepool, getting Return, Flamethrower from level up, and Thunder Punch. It learns Psychic, but you are better off teaching it to something that would benefit immensely from it or for STAB. Iron Tail can also be used, but it is risky with a 75 percent accuracy. This is a sample moveset for it.
Magmar @ Filler
Flamethrower
Thunder Punch
Return / Strength
Fire Punch / Filler
If you have gold and you are gonna use Ho-Oh then here is a set for it.
Ho-Oh @ Filler
Sacred Fire
Fly
Shadow Ball
Earthquake / Filler
If you get lucky and get Entei or if you Mean Look trap it and catch it then here is a set for it. Entei's coverage is pretty weak or inaccurate, so I'd say that Magmar is arguably better, even though Entei has great stats.
Entei @ Filler
Flamethrower
Iron Tail / Filler
Dig / Bite? / Hidden Power (if it's a useful type) / Filler
Return / Strength / Filler
