Easy, normal, hard modes?
Easy = Mudkip
1st = Rock = offensive advantage
2nd = Fighting = Neutral both (offensive and defensive)
3nd = Electric = Offensive advantage and defensive advantage (While ground)
4th = Fire = Offensive and Defensive advantage
5th = Normal = Neutral both
6th = Flying = A little disvantage, but most water Pokémon has Ice beam so...
7th = Psychic = Offensive advantage (Solrock, Lunatone and Claydol has Water Weakness and Xatu can get Ice beamed)
8th = Water = Offensive sorta disvantage, Neutral defensive.
E4
Dark = Offensive advantage (Most Pokémon has Ice weakness or low defense)
Ghost = Disvantage (W-o-W causes problems and they mostly have bigh S.def.)
Ice = Neutral both, but Defense has a little disvantage
Dragon = Offensive vantage (plus Sandstorm imunity)
Steel = Offensive advantage // Water = Depends on oponent, but it has a little disvantage in most.
Normal = Treecko
1st = Rock = Offensive advantage
2nd = Fighting = Neutral both (Treecko´s low defense can be a problem)
3rd = Electric = Defensive advantage, Neutral offensive (Magnemite has a point)
4th = Fire = Both Disvantage
5th = Normal = both Neutral
6th = Flying = Both disvantage
7th = Psychic = offensive Advantage (Same Claydol, Solrock, Lunatone case. Xatu gets a little problematic, but other Pokémon can take care of it)
8th = Water = Offensive advantage, Defensive disvantage (Common ice attacks)
E4
Dark = Pratically neutral
Ghost = ^ Ditto
Ice = Disvantage defensive, offensive advantage
Dragon = If it has Dragon Claw, then it´s good, if not, the it´s not good.
Steel = Disvantage both // Water = Advantage both sometimes
Hard = Torchic
1st = Rock = Disvantage both
2nd = Fighting = Neutral (Torchic´s low Defense and speed is bad BTW)
3rd = Electric = Neutral both
4th = Fire = Disvantage both
5th = Normal = Offensive advantage
6th = Flying = Both disvantage
7th = Psychic = Ultra disvantage (Claydol´s Earthquake, Xatu´s Drill Peck, Lunatone´s Psychic and Solrock can defend)
8th = Disvantage too
E4
Dark = Advantage both
Ghost = Disvantage both (At least it can´t get burned)
Ice = Sorta disvantage, Sealeo and Walrein Surf can cause problems
Dragon = Disvantage both (common Earthquakes and Kingdra´s Surf)
Steel = Just 3 Pokés are Steel typed, the other 3 can cause some problems // Water = Complete disvantage.
That´s why I got Torchic in emerald =D I like Hard games.