The Halo books all get pretty catchy titles: Fall of Reach, Ghosts of Onyx, Contact Harvest, etc.
The games on the other hand are boring ol' 1, 2, and 3. Even the named ODST, Wars, and Reach aren't too interesting. (And I guess Combat Evolved counts, although I heard that it was only used for marketing and Bungie hated the title).
Reach could become "Fall of Reach" like the book, or "Seige of Reach", or "Really Slow Alien Invasion".
Halo would just stay as "Halo", since that's what the game is about.
Halo 2 -> "Betrayals" or "The Covenant" or "The Last Spartan" or "Space Monkey Revolt"
Halo 3 -> "The Ark" or "Last Stand" or "117" or "The Great Journey" or "Cortana Goes Bonkers"
...obviously not the best names, but I like when sequels aren't just number. When I say "The Empire Strikes Back", I don't even need to say Star Wars.
"Dude, have you played Space Monkey Revolt?"
"Yeah, I love the part where you punch a religious leader to death."