I voted for Jack, but really, I love all the characters. Jack stands out for me, though, because he's the classic underdog, a good man who's taken some bad turns, but is essentially decent, and could probably make something of himself with the right tools. The episode where he thinks he has a son is one of my favourites for that reason--with a kid, he feels like he could actually do something with his life. It's quite sweet.
Kate's brilliant as well, of course. She's bitchy and selfish, but when it comes right down to it, she's been screwed around very badly by a jerk of a husband, who she genuinely seemed to love, and she's quite vulnerable--you get the sense she really would be at a loss without Jack and Gloria. There's also hope she could change, just a bit, and be a little more kind to others.
Gloria's all no-nonsense swagger, of course, with enough gumption and practicality for the three of them. She's almost the mother hen of the group, despite her young age. I love her for her endless moxie, and ability to stand up to Kate and give as good as she gets. Really, you couldn't do the show without any of them.