I'd say Po. Ever since KFP 2 he's surpassed everyone in the Furious Five - that includes Tigress - and in KFP 3 he even surpassed Shifu when he mastered chi. We've only ever seen Po use his full power against enemies and he usually needs and incentive to do so, like his friends being in danger or something, but if that power was directed at Tigress for whatever reason I don't think she'd stand much of a chance. In addition, he can learn new techniques on the fly. As of right now, Po is second only to Oogway when it comes to combat. If he wanted to I bet he could take over China without much of a struggle.