There are many good ones! Usually people would say a pseudo-legendary, but it can be a lot of different ones, depending how you take care of your pokemon.
the best fire type legendary is Ho-Oh the best fire type non-legendary is Arcainine
Quagsire
The best non legendary Pokemon, or the best Pokemon in general, is a matter of opinion that depends on your prefered type. Ask anyone and they will give you a different answer. However, the non legendary Pokemon with the highest attack is rampardos, who is not in emerald.
The best legendary fire type is ho-oh. The best non-legendary fire type Pokemon is charizard.
The best non legendaries are braviary, hydregion, and volcanrona
Metagross is the best Non-Legendary Steel Pokémon. He's also Psychic.
Rayquaza is the best, but non-legendary is Salamence.
Best non-legendary water pokemon would be Gyarados. Best legendary water pokemon is Kyogre.
Unfezant and swoobat
Flygon, donphan or rypherior
Depends on your personal preferences. Slaking has the highest average base stats of all the non-legendary Pokemon if you want to base it off of that.
Jolteon.