Exodus, hands down. I love the way the city of New Alexandria looks and I thoroughly enjoyed the more heroic feeling to the mission; helping out UNSC Ground Forces and evacuating civilians. Plus, it was nice to take a break from the Elites and fight Brutes for a while. I really like the way they redesigned the Brutes in Reach, they look much more menacing than they did in other Halos. It was also just really cool to be on the defensive side of the siege, and it also felt more like a classic Halo mission which gave it a distinct feeling from the other missions. It's also really fun and challenging on Legendary.
After Exodus, I'd pick Tip of the Spear and ONI: Sword Base. Tip of the Spear has great intro music and the large push of UNSC forces against Covenant forces is really cool to watch. It was also nice to feel part of a large battle, which is why I like Exodus to. ONI: Sword Base was pretty cool because of the level design and the look of Sword Base, and of course the Target Locator.
I like the mission New Alexandria as well, in part because it's the easiest out of all of them and also because it's really easy to rush through it when going for certain challenges concerning skulls and Heroic/Legendary difficulty where you aren't allowed to die - even once.