Well those actually are all GREAT engines. It is just a matter of personal preference. I would choose the RB WS7III, Ninja or Nova on your list. The C6 is awesome, but I would prefer it in a truggy. I am buying either the C6BB or Ninja very soon also and am very excited...
Right now I am using the RB WS7II x3 which has the same stats as the WS7III but is mainly less durable/ long lasting according to Rody. I LOVE this engine in my 8ight, fast as hell, a little lacking in low end, but that helps reduce wheelspin which equates to faster laps and more control.
OOMSGFX.COM (no longer painting, but take a look for fun!), Roadrunners RC Club
"Racing drivers have balls, unfortunately, none of them are crystal" (David Coulthard)
"I believe the average IQ of a NASCAR fan is only marginally above the things that hit the windscreen during the race. "