I'm not gonna choose a side lol I like them all equally, but if I have to choose and the character + in real life character, I would go with Sam. He's just so great and awesome, he's doing wonderful things now, started a campaign for suicide and those who suffer from it (dunno how else to explain it better lol) because one of his friends died just like that and he started the campaign and raised a lot of money just from selling t-shirts, hoodies etc.. Supernatural fans are awesome :D I also like Dean in real life too.. Damn it I can't choose. Anyway write on youtube Supernatural NerdHQ and watch them both 2014 and 2015 videos, you will love them even more!
Despite being a SuperNatural fan, I don't really take sides with anyone, regardless. From what I've seen it's a dog eat dog, everybody for themselves in a really crapsack world and reality. They all have lied to each other and fucked up in all kinds of terrible ways. All of those characters you listed had fuck ups really really bad in some form or way or another.
And I'm pretty sure Charlie is dead, but she might come back as a guest role or something.
What about Kevin? Poor Kevin. It's sad that he would be killed by an Angel rather than something else and yet he was a Prophet!
Kevin had put his trust in the Winchesters and even Crowley warned him that he is going to get caught in the crossfire and with all these conflicts that the Winchesters are involved in. In the end, Crowley was indeed right. But at least Kevin was able to forgive them and not hold any kind of grudge despite being dead.
The only other character in SuperNatural that had my attention is probably Frank Devereaux, but I think he's long digested in the stomach of Leviathans.
But if he does all of sudden return randomly this season as a guest then I would like to see what he can do to help the Winchesters again. Although I think he was meant for really just a single season like many minor supporting characters.