In my Pokemon Diamond, I have Empoleon, Giratina, Charizard, Mew, Rayquaza, Regigigas, Groudon,and Kyogre (all 100) but I don't know which ones I should put in my team. What 6 pokemons would you Pick and why?

Mew and Regigigas are out of the team. I would choose Empoleon, Giratina, Charizard, Rayquaza, Groudon,and Kyogre. Because Regigigas' Slow Start ability could be a problem because you can't move in the first couple rounds. Mew's status was quite low for a legendary.