: As for Bellicose , I don't think she would fit the bill. Granted her
: exploits under the command of Captain Castilla were "legendary"
: in their own right, and would certainly lead UNSC sailors to tell fabled
: stories of her operations against the colonial forces. However, while she
: was originally commissioned into the UNSC Navy, Bellicose is in service
: with Castilla's rebel fleet.

Since it was considered destroyed after the fight at Theta Ursae Majoris, being a "ghost ship", and holding it's own multiple times against one of the most revered commanders in the Navy, I'd say the shoe fits, even if the rebel-controlled fact smudges things a bit.

Overall, I'd put the Bellicose last, with Spirit of Fire as most likely. I can't think of any other missing ships that might fit the bill.