Definitely Kat.
I always loved Cortana's character, though in Halo: CE the voice acting was poor compared to the next 2 games, and it was too full of techno-babble. She seemed less robotic in 2 and 3, with much more emotion. So by the time the Halo 3 "Cortana moments" rolled around, I didn't mind them at all, because they filled in some plot details and tied in the books a little, while being cleverly disguised loading screens. Don't tell me you never noticed that Halo 3's initial load screens tended to be way shorter than 2's or CE's...
As for Kat, I hate the accent/voice acting. I also just dislike her personality. She seems very arrogant, dismissive, disrespectful.
-She seems to believe she's an amazing soldier when she's the one with the prosthetic arm due to an injury sustained in combat.
-She's disrespectful to her commanding officer (case in point: her sarcastic line "Story of my life" and then "You're in my light Commander")
-Finally, she just acts like a b*tch in Winter Contigency when Six alerts her that he's found something and is handing it over to her. She snatches it out of his hand and growls "Not your domain".
Not to mention her A.I. is abismal....