Dunno, I guess it would be some clan we have not seen yet. If you look at the fight between Sasuke and Itachi, he can summon his weapons very fast from the seals on his arm. And Guy can open the gates in his body to activate chakra.
So if there is parallel universe and you can store up Chakra in there and you can even transfer energy from Myobokuzan to Konoha, then the must be a clan that meditates the whole day to collect chrakra, store it in seals and can activate that in seconds when in fight, so you have a nearly limitless amount of Chakra.
That would be my clan.