What is the best starter in Emerald?

Everyone has a different opinion according to their favorite style.

Sceptile has high Speed and Sp.Atk but low Def and Sp.Def. It has average HP and Atk.

Blaziken has high Atk and Sp.Atk. It has average HP, Defense, Speed, and Sp.Def.

Swampert has high HP and Atk but low Speed. It has average Defense, Sp.Atk, and Sp.Def.

You'd probably want Sceptile or Blaziken, because of their high Sp.Atk. That way, if your starter's battling a Pokemon like Pikachu, its Static ability won't affect your Pokemon if you use a Sp.Atk move.

Actually Mudkip/Swampert would be best for a first time player cause you will have the advantage in a lot of the gyms. (Against Flannery, Watson, Tate and Liza.)

But if you do this I suggest you catch a flying type like Swellow or Altaria cause Swampert is very weak against grass types.