Infernape is the best starter in DPPt, mostly because it has the best offensive typing and highest speed. Grass, flying, and bug resist both of Torterra's STABs, and water resists both of Empoleon's STABs. Infernape also learns powerful STAB moves (namely fire blast and focus blast) before Maylene, and it can learn coverage moves to hit the few Pokemon that do resist its STABs. This is the moveset that I recommend.
Infernape @ fist plate/expert belt/metronome
EVs: 100 Atk
- flare blitz/fire punch
- close combat
- swords dance
- shadow claw/thunder punch
Typhlosion and Feraligatr are roughly equally good, even if one of them is a bit better. Both of them have a high attack stat and can outspeed most opponents. The biggest differences are that Typhlosion learns powerful STAB moves (namely flame wheel and fire blast) earlier in the game, while Feraligatr has better coverage moves later in the game. Feraligatr might also be outclassed by Gyarados (it arrives at a higher level, has higher physical attack, and learns dragon dance), while no other fire Pokemon in HGSS is nearly as good as Typhlosion.
Typhlosion @ charcoal/choice specs
EVs: 100 SpA
- flamethrower
- focus blast
-dig
-return/hyper beam
Feraligatr @ mystic water/metronome
EVs: 100 Spe
- waterfall
- ice fang
- crunch
- flail/earthquake