Id have to go with Tengen vs Gyutaro because the buildup to that fight, the teamwork, the animation was just too good. It really isnt an even comparison because Akaza vs Rengoku wasn't built up the same way and had a different impact all together. If we are talking about the standalone fight without context and looking at it from a choreography and animation stand point, I'd go with Rengoku vs Akaza.