Crap. I just stared at the list of names and kept reasoning out each character.
Saul: Get the fuck out if you think he is the rat.
Angel: I thought maybe yes, but figure that he is greatly invested in The Tower. Why the hell would he want jump ship? Despite the possibility of some tie between he and Scratch, I don't think it's a friendly one. You can tell he hates that ...witch.
Kelly: She was drunk at the time and having a blast. She may have called the Mallers earlier, especially since Tommy was on duty and she could have bullied the kid into not mentioning shit. Plus she is a lawyer and unless she was a prosecutor I'm willing to bet that she has had relationships with criminals, atleast on a professional level. At the time this all was going on she had a rocky relationship with Michael, but she did support his/Peggs idea of throwing a party which meant that she possibly knew the Tower would be vunerable to attack so she could call her buddies at the mall and tell them to attack...shit! I actually have no reason to say Kelly isn't the mole. Sure, she's been cool as shit lately, but at the time...who knows?
Burt: Stfu, do you really think he has a pact with ol' Scarface?
Bill: Very possible, though he was too uneasy about trusting people in his own building. What makes you think he's going to contact former inmates? He was hording food and ammo, but I think that was for selfish reasons, not to take away from the Towers defense when the Mallers attacked. Plus he went out fighting which says a lot about his stance with the Mallers.
Pegs: Nah. She's just too sweet and adorable to do something underhanded like that. Plus she's numero uno on Scratch's shit list. She called the Mallers on the CB just so she could kill Latch? Yeah, that's an awesome theory. Right up there with Johnson having Kennedy killed just so he could finally be president...
Lizzy: Nope. I don't think she's a fan of being raped. Latch and Scratch seemed to imply that they wanted to enter a brother and sister tag team match with Lizzie. You don't have to be a psychiatrist to figure out how she would feel about that.
Riley: Hmmm. I don't think there has been any motive for Riley to be the rat. Unless she is really that sick of "Peegz"
Datu: Seeing and hearing Samantha torn to shreds really screwed with his mind for a while. He may have tried to drop the A-bomb on the tower by contacting the Mallers, but it's doubtful. It would be a hell of a kick in the gut to learn our sweet little Datu is the mole, but I don't think he is. Things were going rather well for him at the Tower, and he appears to consider the other survivors as his family. Even after the death of Samantha I don't think that Datu would have turned into such a fatalist that he'd betray the Tower.

Then you have people like Steven and Kalani. Kalani is shady and I'm not quite sure where I stand on him being the mole. I lean towards no because he's our pilot. He's going to be the one to fly everyone the fuck out of there, then die. As for Steven, well I'm not sure yet because we've only seen him be an asshole recently. Before this he was just some other survivor. Who knows? Maybe we'll find out he was the bastard that fired the infamous first shot; the shot heard round the tower!

So at this point, I kind hate to say it, but I'm leaning towards Kelly being the mole. She may be carrying the burden of knowing that she was responsible for Tommy's death and that is why we've seen such a major change in her character. That's my guess.