[PkmnMstr]

Given that the riders would actually want to fight, I would assume that they would fight honorably and give it their all, because that's what warriors do. They tend not to use dirty tricks. So I don't think transformation times and junk like that would factor in as heavily as some people are making it.

And then the whole trinket transformation thing. I've always operated under the assumption that the switching of trinkets was purely for dramatic effect and selling of the gimmick for merchandise purposes. So in reality, any required time would be negligible.
Besides, if enemies are always giving them time to switch, if these battles were actually produced and televised and stuff, what's to say riders would not give each other that same courtesy? It goes with the whole fighting with the best ability thing.

That being said, I really like Kabuto even though I have yet to see the show, but my vote goes to OOO. I could just imagine Eiji combo spamming a la the Gokaiger's Gokai Change-a-thon(and they seem to be able to change seamlessly despite needing keys and a bulky phone changer[goes with my negligible change time argument]) and using all sorts of various attacks against Tendou's martial arts.