Red is clearly the strongest Dex holder. And for those that mention the battle against Bruno (The 1st time), that was Lorelei that mainly defeated him. Even Gold, who manages to finally defeat The Masked Man, asked him for training to beat Silver, who, to be fair, is a carbon copy of Green (Girl), despite the fact that they were joined together since the very beginning.
Blue is also rather good competition - To be honest, I'd be looking forward to an epic battle between the main protagonists (Red, Gold, Ruby, Pearl & Black) against all of their rivals (Blue, Silver, Sapphire, Diamond and White).
I also think it would be good for all of the female counterparts to have an epic battle, or something along those lines.
But yea - Red all the way. ;D