Which one of any of them? For the modified format, there's the CG Groudon and Kyogre along with the NP promos, and then there's the unlimited HL Versions. And as for Rayquaza, there's the delta one in DF and the regular in DX, also along with an NP promo and an unlimited card in DR. If I'm not mistaken, there's also older promos for both Groudon and Kyogre EX.
If it's all of the above, I'd hafta go with Rayquaza EX d. None of the other modified ones are that bad either... Kyrogre and Groudon NP are both great techs also with so much Destiny out there, and the CG ones are good in decks with things like Meganium EX if you feel Latios* isn't enough. Kyogre EX is a great sniper and doesn't get in the way of your start. Ray (DX) is alright, especially with Firestarter Blaziken back, but Rayquaza* is just as good.