Mr. Onasi. Hands down.
First of all, he is a man of great loyalty, honesty, and bravery. He knows where he stands. In the first ten minutes of the game, he's saved your PC twice at great risk to himself. He is a pragmatic lightsider (as evidenced with the bounty hunters and the old man, the bounties from Zak's office, and his "I'm glad to see we're still the good guys here" line when you act charitable). He's still devoted to the memory of his wife and son, trying to get Dustil back from the Sith. He is man enough to apologize to Mission and to want to take care of her. He's not the best fighter by a longshot, but he's got enough heart to make up for shortcomings there.
Atton changes his loyalties more frequently than his shorts. Let's see, betrays the Republic. Then, betrays the Sith. His almost-casual tone when he's talking about how much he hurt that Jedi woman and how much he enjoyed it reminds me of a sociopath I had the misfortune to encounter. Then Kreia has to mind-rape and blackmail his sorry hide just to stay with you. He admits he's hoping you get him killed, and it's implied that he was Coorta's contact, meaning he was the one about ready to hand you over to GOTO. And he's a compulsive gambler and borderline drunk on top of all those.
Carth only crashed the ship once. Atton crashed damn near everything he flew.
Atton's special ability was handy, but his stats bit hard. Carth had better balance when it came to stats and more staying power in a fight. I do not count the Force-sensitivity either way, as there are quite a few in-game hints that Mr. Onasi is also FS.
Atton killed a Jedi "because he loved her," and tells the whole story of how he tortured her in a very creepy near-monotone. He also acts like a sleeze towards F!Exile. Combined with his utter inability to stay loyal to much of anything? If he asks for your number, run like hell.
Carth? Faithful to his wife, and faithful to her memory after she's gone. When he does come out of his shell, he's respectful, if a bit fond of kidding around ("Hairless Wookiee"). Hell, his K2 speech even indicates he's faithful to Revan (either gender, but Sbarge really kills you if you say Revan was LSF) four years after she vanishes. In the DSF ending, he flees because he knows he can't fight, but he makes a last-ditch attempt to march in and save you from yourself, knowing he's dead either way.
It's not even a contest.