Sorry, coming to this a little late.
I've heard good things about the Halberd destroyers, although I don't yet have any. The rules for the Valiant-class are supposedly being reworked, right now it's pretty much a different-looking Marathon (stats are near identical).
The Orion assault carriers are pretty cool, they deliver a pretty hard punch and have different options for how you field them.
But I would also tell you to get more Marathons. They are the workhorse of the UNSC fleet (as it stands right now).