For starter chat, Gen3 is actually really interesting because the best in the end, Torchic, is horrid in gen 3, taking the role of grass for the first 2 generations. As a base analysis.
Torchic Fire (w/ Fighting & Flying moves)
- Preforms well on 3 gyms (Rock, Fighting, Normal)
- Preforms acceptably on 2 gyms (Fire, Electric)
- Preforms poorly on 3 gyms (Flying(with Ground), Psychic, and Water)
and to top it off, Torchic is horrible against both teams, being weak to Aqua and resilient, not powerful to, Magma
The primary concentration of poor performance occurs in the second half of the game, during the Water Areas, the Primary Aqua Magma Arcs, and the Final 3 gyms.
to compare, Treecko and Mudkip starts
Treecko Grass
- Preforms well on 2 Gyms(Rock, Water)
- Preforms acceptably on 3 Gyms (Normal, Fighting, Psychic)
- Preforms poorly on 3 gyms (Electric, Fire, Flying)
Treecko preforms excellently against Team Aqua, and conversely suffers the most against Team Magma.
Treecko preforms best in the late game, but lags in the mid-game where it lacks the moves to deal with the 3 gyms. It will also preform poorly when you encounter Magma the second time due to their acquisition of actual fire types.
Mudkip Water&Ground (with Meme type attacks)
- Preforms well against 2 gyms (Rock, Fire)
- Preforms acceptably against 5 gyms (Fighting, Electric, Normal, Flying, Psychic)
- Preforms poorly at 1 gyms (Water) (note: Mudkip doesn't preform too poorly compared to the other mons in this category, but one of it's semi-guaranteed moves of it's moves are useless, so applies)
Mudkip Preforms well against Team Magma, and resists Team Aqua
Mudkip preforms acceptably throughout the entire game, and can be used as a Surf/Dive/Waterfall user, so has that. It lacks much power outside of it's TM set though, so take it's type effectiveness with a grain of salt.
Granted, with TPP style, Torchic and Treecko, who usually preform worse than mudkip in analytical sense, actually would do better because they learn better moves than Mudkip. Of the two, either could preform well during the game.