The mainline Halo series has a long history of excellent and memorable opening campaign levels. Nearly all of them do a great job at setting up the story and introducing the player to the type of gameplay that will go on to shape the experience going forward, whether it be single or cooperative.
Some games in the series can certainly be argued as being better in this regard than others (as will become abundantly clear), but Halo's illustrious reputation as one of the premier campaign FPS games is generally due in no small part to the strength of each game's opening missions.
8 Halo 5: Guardians – Osiris
The introductory level of Halo 5's campaign is a rather silly one in many ways. Gone are the tense and tactical set pieces of previous games, instead the game chooses a bombastic, avengers-like approach as an introduction for the brand new, Fireteam Osiris.
The mission is somewhat of a disappointment because not only does it fail to establish the characters of Fireteam Osiris, but fighting both Prometheans and the Covenant is as frustrating and boring as ever. The mission also ends with a big let-down in the fantastically unceremonious death of Jul 'Mdama.
7 Halo 4 – Dawn
Dawn is probably one of the best levels in Halo 4's campaign for the simple fact that it has such a traditionally "Halo" feel to it. In addition to finally playing as Master Chief for the first time since 2007, the level does an excellent job of setting up mystery and tension as the player fights a Covenant splinter group aboard the destroyed ship, Forward Unto Dawn as it is pulled into the Forerunner shield-world known as Requiem.
While the later parts of the game lean too heavily on linear design and (admittedly poorly designed) Promethean enemies, this first mission benefits from the narrowed scope and focused gameplay direction as it serves to familiarize the player with the new without overloading them in the process.
6 Halo: Infinite – Warship Gbraakon
While Halo: Infinite fancies itself as more of an open-world game than its predecessors were, it begins similarly to many of them; with a tighter, more confined mission set on some kind of spaceship. Granted, the level is a fair bit more open than those previous games' levels, and that has to do with the rest of Infinite's experience being open-world.
In this way, the mission does a great job of teasing the more open-ended aspects of the game without sacrificing fast-paced, and tense action set pieces that come with more linear gameplay.
5 Halo 2 – Cairo Station
Cairo Station is a fan favorite level in Halo 2 for a number of reasons. First and foremost, there are no Jackal snipers to be found in this level. The second reason is that it's a tightly paced (though noticeably linear at times) affair that delivers a number of unforgettable moments, such as the "I need a weapon" scene or giving the Covenant back their bomb.
In addition, it feels like simply a more intense version of Halo CE's opening level, with more diverse encounters, new weapons, and bigger set pieces along the way.
4 Halo: Reach – Winter Contingency
Reach has probably the largest and most openly designed first level of any Halo game before the release of Halo: Infinite. There are even vehicles for the player to use on more than one occasion if they wish. Aside from this, what really makes the mission fun and memorable is how dynamic it is. It starts off somewhat tense and mysterious, only for the Covenant to launch a surprise ambush on the player's squad. Intense fighting across a large and open valley ensues as Noble Team moves closer to their objective.
Along the way, the player is given plenty of opportunities to learn about each member of their squad. Each member's personality/character, weapons, and abilities are put on display for the player. This is something that Halo 5 struggled with in particular, whereas Reach more or less succeeded.
3 Halo 3 – Sierra 117
The first level of Halo 3 has quite a few things going for it. Firstly, the introduction of the Chief + Arbiter partnership is great, secondly, the environment design still holds up to this day almost 15 years later, and thirdly, it is an excellently paced mission, beginning with tight jungle combat and gradually opening up as the level progresses.
This gives the player more room to play how they want and experiment with the new weapons and equipment, but in a completely organic and endlessly fun way, making the level feel less linear. The tone is excellently set up, and the player is, near perfectly, given a taste of the greatness yet to come (except for the Cortana flashes).
2 Halo 3: ODST – Mombasa Streets
Mombasa Streets is easily the most unique opening level in the whole series. Not only does it serve as the hub world for the player in between later missions, but Mombasa Streets is also bathed in a level of atmosphere and design that no other Halo game has been before or since. Usually, Halo games start with an exciting set piece to draw players in from the get-go, but ODST slows things down almost mediately as the player is left to wander the dark, neon-lit streets of New Mombasa alone, surrounded on all sides by Covenant invaders.
Bolstered by an excellent musical score, Mombasa Streets takes on a somber, neo-noir tone as the Rookie attempts to navigate the maze-like city. ODSTs are not Spartans and are thus, weaker against certain enemies. Along with the new suppressed weapons, navigating this city is a difficult yet rewarding endeavor.
1 Halo CE – Pillar of Autumn
What more can be said about the first campaign mission of the Halo series? This is a level that not only needed to introduce the player to brand-new characters, weapons, and enemies but to an entirely new universe as well. Pillar of Autumn manages to do this perfectly and be an insanely fun and fast-paced level at the same time.
This is the level that built the foundation for nearly every other opening mission in the Halo series. It beautifully sets the tone and provides a tightly paced, action-packed set piece to get the player accustomed to the game's mechanics while drawing them into the world and characters in a natural and memorable way. This probably sounds familiar to fans who have played the sequels, but this is the game that set these trends for the series.