Hurrrg, nope, can't do it. Even aside from the whole Cassie-hate debate, I've had each and every main character as my 'favourite'.
Cassie because she's the moral compass and the empath -- she was particularly my favourite when I was younger, but also I LOVE #19 and #29. At the same time, she got some of the worst books, too.
Marco because he reminds me the most of me (currently -- I disliked him as a kid, thought he was an annoying jerk.) Also #15 is one of my top 5. Still, some other books of his are just... not memorable. He seems to get a TON of action books, which might be why I prefer #15, which is more emotional and internal through much of it.
Jake absolutely fascinates me; #47 is my favourite book of the series, and I love all it says about his character. I know he was bland at first, but that's part of what makes him interesting, if that makes any sense.
Ax I've always loved because, like Marco, he has the capacity for great comedy and great drama. I find his serious moments extremely powerful, and he has some of the most growth as a character. All of his books were memorable to me.
Rachel was my other favourite as a kid, and I still love her because, while she does become more and more a Blood Knight, it's a very realistic transformation. Still, she got a couple of my least favourite books, which is a shame.
Tobias is an on-again off-again favourite of mine, and as a kid, #23 and #3 were some of my absolute favourites. His tone is very distinct, and he becomes one of the first voices that you'd recognize without being told who it was. I like the more distant, observant pacing of his books and I love the hawk elements.