Number one has to be Hurley. As Sayid picks him up at the mental hospital and in the ensuing mayhem, he's scared, he's frustrated, he's confused, he's guilt-ridden. Just like you'd think a real person would be. He's not a bearded automaton like Jack who seems to have all but had his personality wiped out, and he's not a blubbery mess like Kate. He's not gone hardcore like Sun or jaded like Sayid. He's still Hurley. His scenes with his mother and Ana Lucia were welcome and provided both comic relief and tears. Jorge Garcia is just getting better with each season.
Number two is Sawyer. He is also reacting like a real person: scared, angry, devastated by what he thinks was Kate's death, confused, and looking for someone to take it out on. This is not the user and con-man of early seasons, nor the smooth talker and lover boy we saw later with Kate. This is Dark Sawyer and he's been in the making since he killed Original Sawyer in The Brig. He's still funny, but with a definite edge. And he still feels real. He doesn't act as though he's an action hero, he acts like a real person.
And number three is Juliet. Women of Lost seem to either die (too many to mention), get whiny (Kate) or made irrelevant (Charlotte and Rousseau, before she died). Right now Juliet is acting like a human being, not a foil for male characters. She's taking initiative, she's helping people, she's trying to figure her way around the new reality of time-skipping. And she's babysitting Sawyer, who really needs it.
There's a nice chemistry between the two, kind of like an older wiser sister/younger foolish brother thing. Whether it will be forced into romance remains to be seen... I kind of hope the show let's them just be friends.
So far I haven't really seen enough of any other losties to judge them I guess. Locke is still Locke and awesome, though he's getting more action than drama atm. And the 3 freighters - Daniel, Miles and Charlotte - are kind of following the paths they started on last year. Daniel explains what's going on, Miles being all sarcastic and psychic, and Charlotte is unfortunately not being used for much at all except a foil for Daniel.
And then there's Alpert. But he's almost more plot point than person. Which is ok when you have eyelashes like that.