I'm thinking Manaphy and Kyogre Tag Team GX.
Onix and Groudon Tag Team GX
Alakazam and Deoxys Tag Team GX
Darkrai and Yveltal Tag Team GX
Gardevior and Xerneas Tag Team GX
Latias and Latios Tag Team GX
Cresselia and Palkia Tag Team GX
Metagross and Dialga Tag Team GX
In my opinion.
Here's a question that boggles the mind... What exactly would Magikarp do to actually support a Wailord in a tag team? Is it like Wailord's good luck charm that encourages it to do larger splashes or something? Or maybe the Magikarp is the head honcho brain in its support... Just imagine the headcanons here! xD
Welp! Time to proxy up a Quagsire/Naganadel/Wailord-Magikarp GX deck. Throw in some Tapu Koko spread, Volcanion Prism, Kyogre, Aqua Patch, Beast Ring, and Beast Energy, then one big GX attack for possible game.