For me it's a toss-up between Jorge and Kat.
For his short time in the campaign, Jorge left a significant impression, being one of the few to treat Six like an actual teammate rather than a replacement, and also for being the only Spartan II among Noble Team. In the end, he died thinking he'd saved his adopted homeworld, only for an enormous Covenant fleet to appear and show his death to be in vain.
I find Kat's sad because it comes out of nowhere and it's so unexpected. We'd spent a good amount of time with her since the beginning of the campaign, and she was one of the most visible members of the team. The moment she gets sniped by the Field Marshal, for me, is the moment that it really sinks in that Reach is doomed. And that doesn't do it, then the long shot of the glassed city certainly would.