[Starscreamer]

I think Dark Kiva is moot...but Ouja, I didn't cast because of the "Skull factor" per say...he has the intent AND the means. Especially if he has all three beasts, he does use them seperatly. When he uses the Advent, the beast comes pretty much from nowhere, he blindsided many people with the Rhino contract monster, and Evildiver does attack from above. Not to mention he has all the other cards, so he can use the strike vent and Swing Vent as well. One of my favorite seens in Ryuki is him just walking down and using beast after beast on different Riders. And yes the desire to kill is a VERY powerful thing. Ouja would open a battle with a final vent if he was "vexed."
Agito, is a powerhouse, but you have to get him really ticked off before he hits the higher levels of power.
Ryuki? not even going to go there...
New Den-O would underestimate him amd get mowed over by the variety of attacks
Zeronos...ok, I would pop the popcorn and watch this...he would make it a good fight, yes. Even with Vega form, but Ouja does know how to actually duck from a ballistic attack. I envision it being Ouja seemingly knocked back, even backed into a corner, but Venosnaker looms from behind and...well...bad things happen to Zeronos...
Dark Kiva though would fight sneaky, and never hold back. Ouja would just have to fight dirtier than usual.