Statistically...
Treecko, when fully evolved, is by far the fastest of the three Hoenn starters. It is a close second in regards to Special Attack and Special Defense. Its Attack, Defense, and HP Stats, however, are the lowest among the starters.
Torchic, when fully evolved, has the highest Attack and Special Attack. It has the lowest Special Defense among the three, while it is in the middle for the other three stats.
Mudkip, when fully evolved, has by far the highest HP stat, as well as the highest Defense and Special Defense stats. Its Attack stat is high, and its Special Attack is moderately high, but it is by far the slowest of the three.
So, the best starter depends on your strategy. If you want something that can hit hard and fast, Treecko is your best bet. If you care about hard hits more than anything else, Torchic should be your choice. And if you want a Pokémon that can take a lot of hits and then hit back hard, Mudkip is for you.
Of course, you could also compare it to the Gym Leaders and the Elite Four.
Rustboro Gym: Rock; Treecko and Mudkip both have type advantages here, but Torchic will have a hard time unless you manage to evolve it before you face this Gym.
Dewford Gym: Fighting; Treecko will be resistant to Fighting-Type attacks, and Torchic will have an advantage if it has learned Peck, but other than that, no starter has a real advantage here.
Mauville Gym: Electric; Grovyle (Treecko's evolved form) will be resistant to Electric-Type attacks, and if Mudkip has evolved into Marshtomp by this point, it will be immune to Electric-Type attacks, giving it the biggest advantage in regards to Type. Torchic or Combusken will have an easy time with the Magnemite, however, so all of the starters are good here.
Lavaridge Gym: Fire; Marshtomp has the best advantage here, especially if it has already evolved, since Water and Ground are both strong against Fire, but Combusken will also do well, since it can resist Fire and attack with Fighting. Grovyle, obviously, will struggle most here.
Petalburg Gym: Normal; while Blaziken (Combusken's evolution) has the Type advantage, this Gym revolves around strategy, so it would be just as effective to utilize Swampert's (Marshtomp's evolution) endurance or Sceptile's (Grovyle's evolution) speed.
Fortree Gym: Flying; Sceptile has a type disadvantage here, as does Blaziken due to it being part Fighting-Type, though its Fire is strong against Skarmory. Swampert's Ground-Type attacks are useless against the birds that this Gym uses, and Water isn't strong against the part Dragon-Type Altaria or the part Water-Type Wingull and Pelliper. All in all, this is likely to be a bit of a struggle for any of the three starters.
Mossdeep Gym: Psychic; Blaziken has the type disadvantage here, but could be useful nonetheless, as Psychics typically have low Defense. However, thinking along those lines, Swampert is likely to be the best bet here: capable of taking hits, strong physical attack, not to mention the fact that three of the Leaders' four Pokémon are weak to Water. Sceptile would fare moderately well, though it may struggle with the Xatu.
Sootopolis Gym: Water; Sceptile obviously has the type advantage, but can be endangered by the fact that many of the opposing Pokémon know Ice-Type moves. Blaziken has an obvious disadvantage, though its Fighting attacks would prove effective more than once. Swampert would fare moderately well; it does not resist Water or Ice due to being part Ground-Type.
Elite Four Sidney: Dark; Blaziken is the best choice against Sidney, due to Dark being weak against Fighting and two of his Pokémon being part Grass-Type, but the Sharpedo may prove difficult. Sceptile would likely be a good choice also, but Swampert may have difficulty against Shiftry and Cacturne; Water and Ground are both weak to Grass, meaning that their attacks would hit four times as hard as usual.
Elite Four Phoebe: Ghost; none of the three have distinct advantages, but Blaziken has the most disadvantages; many of Phoebe's ghosts know Psychic attacks, and Fighting-Type moves cannot harm Ghost-Types. Sceptile and Swampert would likely fare moderately well here.
Elite Four Glacia: Ice; at first glance, Blaziken would be the best choice here, due to Ice being weak against Fire and Fighting, but since three of her five Pokémon are part Water-Type, all three starters are likely to struggle.
Elite Four Drake: Dragon; Blaziken has the most disadvantages, but no well-trained Dragon-Type is easy to defeat without Ice. If Swampert has been taught an Ice-Type attack, it would be best here. Sceptile would be at a disadvantage due to Grass being weak against Fire-breathing Dragons.
Champion Wallace: Water; Sceptile, again, seems to be the obvious choice here, but while Wallace has no Ice-Types, three of his Pokémon know Ice Beam or Blizzard, including a Tentacruel which would be particularly threatening due to its Poison-Type. Appropriate for the Champion, no Starter has a perfect advantage.
You could also look at how abundant the Grass, Fire, and Water-Types are in the game apart from the starters. Fire is by far the scarcest of the three, and Water is the most abundant, if you have Rods and Pokémon that can Surf.
i think that mudkip is the best starter because:
he/she helps you through 1st gym,
3rd gym, 4th gym,
elite four,
steven(x2),
you don't need to catch a Pokemon for surf
and raquaza.
torchic is a good starter too for 5th gym but so is mudkip so is treko cause it saves you money and is very strong but this is my opinion if you want to choose others then go ahead this is just for you to read
Suicune is not a possible starter Pokemon in emerald you must get a suicune from Pokemon firered or leafgreen.
The grass is the best, because in the end, you will need him a lot! It is though only based on preference (But i heard you like Mudkipz)
Mudkip is a starter Pokemon on Pokemon Emerald, Sapphire and Ruby. This means that it is the first Pokemon you get. You get starter Pokemon near the beginning of the game from Professor Birch. Other starter Pokemon are Torchick and Treecko.
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
-Azumarill -Magnetric -Breelom -(A flying pokemon) -(a normal, fighter or psychic)
Everyone lieks Mudkipz!
Suicune is not a possible starter Pokemon in emerald you must get a suicune from Pokemon firered or leafgreen.
The best starter Pokemon in Emerald/Ruby/Sapphire is Torchic. If you level it up well, it'll sail you through the first 3 gyms with ease.
The grass is the best, because in the end, you will need him a lot! It is though only based on preference (But i heard you like Mudkipz)
Mudkip is a starter Pokemon on Pokemon Emerald, Sapphire and Ruby. This means that it is the first Pokemon you get. You get starter Pokemon near the beginning of the game from Professor Birch. Other starter Pokemon are Torchick and Treecko.
You cannot get all 3 Starter Pokemon in Pokemon Emerald without trading for the other 2.
Torchic, it can learn Sky Uppercut which attacks even Pokemon that used Fly
-Azumarill -Magnetric -Breelom -(A flying pokemon) -(a normal, fighter or psychic)
no
you have to get it as a starter to get it
the best is either mudkip or torchic because of their evolutions mudkip= swampert torchic=blazekin
in my opinion they are rayquaza,ditto,latios,your starter pokemon,aipom and anorith hope i helped