I'd pick one of two options: first, the Star Wars geek in me has to say a Super Star Destroyer. Those things are MASSIVE. Certainly bigger than, say, the Covenant Supercarrier I saw mentioned. They're incredibly rare because of how immensely powerful they are. Second choice, is the UNSC Infinity. I think a ship that's big enough to carry its own accompanying FLEET would be a solid choice. Having a full deck, on a ship that massive, dedicated to nothing but Spartans, as well? Oh man. Cabal would have their hands full.