Cap, in a stomp. Yes, even without the shield. Not only is he a better fighter than Danny, but honestly, the Iron Fist technique itself was never that impressive. I can't see it achieving any kind of magic one-hit KO, not against someone as durable as Cap. He'd feel it, but he'd then get back up, keep fighting, and win anyway. And that's assuming no shield.
I don't really see Bucky losing either. He's basically got the same physical advantage, and while he's not quite as skilled as Steve, he also has a super strong arm as a weapon. It evens out.
If you want an even fight, you'd need. . . hmm, maybe Black Widow? She has numerous gadgets and weapons, but she also has to actually worry about Iron Fist tricks.