This is really tough because they've introduced some fantastic characters in the extended universe. It's hard not to go with one of the studs on Fireteam Osiris but my favorite Spartan IV is a clear fan favorite - Gabriel Thorne. We saw so much development of his character through Spartan Ops and Escalation it's hard to not feel stronger for him than the other Spartans. He's every characteristic of a good soldier which is a wonderful setup for him to possibly make some questionable decisions against ONI in the future (he's already questioning some of the things they've done).
I'd love to see some more Fireteam Majestic in the future.