Alright, easy. in order from greatest to “heh, I love this guy’s lines”. Though it may be biased, because I can’t unsee the way the books showed me some of the major character’s more subtle nuances.
The arbiter. Hands down. The definition of a fall from grace. I mean, he was one of the most powerful mother fuckers in the covenant (in his division), and in the blink of an eye he is disgraced, and the whole civilization wants to see him dead. Then, ironically, the faction that he despised the most, and the faction he was accused of being involved with (the heretics) was RIGHT about the covenant religion being false. Kieth David’s voice talent is also a plus.
Cortana. An AI that’s attracted to a human. To make matters worse, she’s a copy of the mind of someone who was a MOTHER figure to that same person. That’s fucked up. It also makes her character REALLY interesting. Also, Jen fucking Taylor. I mean come on.
Sergeant Johnson to a lesser extent. Just for cool factor. Also, (you saw this coming) I’m a fan of David skully.
Sgt. Pete stacker. He’s not a major character at all. Fuck, he’s named after the guy who does his voice, and his face is copypasta’d from the director of campaign content. Why is he awesome? His fucking lines. They’re just too damn good. Most are improvised.
This is all besides the point though. As said before, Halo’s story rides on events , with characters just there for exposition. Half life’s story is the same, but it’s emphasis on character is DISTRACTING from the story. No one in the game is really needed to the extent that they’re used.
Also, how could anyone possibly gather that this has ANYTHING to do with TF2? If anything, it’s more like shadowrun… But that’s because Sage Merrill was the lead (forget title that goes here) designer for both.