Halo: The Master Chief Collection (Halo: Combat Evolved and Halo 2)

Right now, The Master Chief Collection is the premiere Halo experience on Xbox and PC, full stop, and it runs damn near flawlessly on the Steam Deck. As Halo: Combat Evolved and Halo 2 came out in the early 2000s, these games are a cakewalk for the Steam Deck, with one major, huge, essential caveat: use classic graphics if you’re trying to save battery life. Playing this way, you can expect a solid three to four hours (if not more) on these games, especially Combat Evolved
Actually, as far as Halo: Combat Evolved is concerned, you should always use the classic graphics as the newer ones—pretty as they are—have such different artistic direction that it robs the game of its original character and charm. Both Halo: CE and Halo 2 let you flip between the two graphical presentations with a press of the View (select) button. But you can also set the audio and visuals to the classic mode in the campaign launch menu.
In my experience, Halo 3, ODST, Reach, and Halo 4 push the battery a little too much to be fully recommended for Steam Deck play. They all run great on the Deck, but not for super long. So if you’ve got time to kill, shootin’ at the Covenant through the first two entries of this series ought to do the trick.