Kyogre, even though the above post is correct about the solarbeam but you've got to take in account the amount of water moves that Kyogre has compared to the one grass type move Groudon has. It also depends on what stats they have, I'm pretty sure Kyogre would win with STAB advantage over Groudon and Kyogre looks cool. I'm pretty sure Kyogre would be faster than Groudon.