i wouldn;t count marco out so quickly. in fact, i might out my money on him. from reading this books/narrations, marco seems the most intelliegent and cunning when it come to dealing with people and situations. i think he is also the most 'street smart' out of them. although tobias had it rough and tough, i don't think he is a street smart as marco, or as cunning.
so, these qualities would make marco win imo. cassie would go down first cuz shes way too much of a sentimental softy, her emotions could take over causing her to hesitate etc.
Jake is the wise leader, but like rachel, he can be a hothead at times and this is a flaw that could be exploited. jake excels in teamwork, that is, he makes his decisions based on the input of everyone elses. his decisions on its own aren't that great, it is with the input of the others that gets it modifed and becomes great. when alone, as an individual, he ain't all that. nowhere near marco's cunning intelligence.
Rachel i think would go down after cassie. sure, in a head on fight, shes like a bomb, a hurricane, and if the oppenent was the same way, and they were fighting in a ring no cheating, purely fire power vs fire power, then yeah, rachel could and probably would come out on top. but facing a cunning opponent like marco in a no rules anything goes battle, there is a significantly high chance that rachels temper and impatience would get the better of her and she would simply rush into the fight without thinking and someone like marco could set traps for this.
tobias, with his experience at being a predator, would be the other one out of them who is most likely to come out on top, besides marco. him and marco would make an interesting battle of wits and instincts and strategy. tobias, due to him living as a bird of prey, would be ever alert, on the guard, careful and able to spot the minor details in things easily. he would be a hard one to catch of guard.
but however, none of them is perfect. is we see, they all get into trouble at one point or another individually. it is when they work as a team that they overcome it. k.a has done a good job in having each of her characters complementing each others strengths and weaknesses.