Jirachi, Reshiram's weakness is Water, so you want something that can cover that. Your biggest threat in that area will be Kyurem and its EX - those are weak to Steel, so Cobalion would be appropriate. You can cover his Fire weakness with a Water-type like Kyurem/Kyurem EX, whose Steel weakness is in turn covered by Reshiram EX.
Congratulations. You just looped back to the start again.
Mewtwo EX is just great in general for heavy hitting, as is Shaymin EX late game. Regigigas EX is okay, but he's situational - then again, in a Damage Swap deck, he probably feels right at home.