That is certainly the best, most influencional and remarkable game, as much as a game itself as a FPS.
Gameplay:
The gamplay it's simple and on a arcade FPS style - when I first played, I found it really weird that the game don't have sprint - you can walk, jump, crouch, use melee. It's that basic and it's really good. Where the Gameplay really shines is in the variety of things you have and things you can do , either on campaign or in the Multiplayer. The variety of Weapons, Vehicles and Maps is really good. It's also one of the Halo games with most variety of Weapons and Vehicles differents and represent their respective factions, that is the Humans and the Covenant.
But, I think it's important to say, that some of the newcomers can find the Gameplay - more about the movement - really slow. As I said, it's a really basic, simple and arcade style of movement; you don't have sprint, double jump, dash, wall running, or neither of this thing that the more modern Shooters have. But this don't make the movement bad, it makes the game, kinda of, more tatic because you can't just go running arround and get killed instantly. Makes the players think more carefully in the Multiplayer, have to "study" the map to know more routes and improve.
Also a good point on the gameplay about the campaign it's that the game gives plenty of options to the player on how to play. Isn't just tons of enemies shooting at you and you have no need to be on cover all the time. You can choose how to pass by the levels, also by the really good level design that gives that freedom. The enemies have different functions and ranks that make you choose wich one kill first to make it easier. You can pretty much be careful, tactical or just rush into the enemies, just use the strategy that you think is the best. This also gives the feeling of you being the supersoldier that Chief is.
Campaign:
Story:
The story is one of the most epic, as every story of Halo should it be. But it lacks in some parts to make the story a little bit more interesting. Maybe the story can be just too simple, and didn't have that much things to put and reveal because they already showed a lot in the first two games. Some characters have their highlights, Sgt. Johnson and Miranda Keyes for example, but it could had been more. Arbiter for example turned basicaly into just a AI that helps Chief in the campaign, even tough he also receives a lot of highlights even in the start of the game. The Covenant also was more intimidating in Halo 2. That game packs more on the character development and dialogues.
Sandbox:
As I said, Halo 3 is the one with most variety of Weapons and Vehicles. And in the campaign you can find plenty of these to play as you want. That is also one of the good points of the game, it really give some freedom to the player to play as he wants. The maps aren't just linear paths, you can explore to discover some "secrets" like Skulls and Power Weapons.
Also have a interesting pattern and variety of enemies, it's interesting because you can see that isn't just tons of enemies shooting at you. You can see that have some kind of organization in each squad and Ranks. In the games you have Brutes, Grunts, Jackals, Flood, that have different forms; etc. With the Covenant squads, Brutes have different types of armor that show their Ranks and how their squad can be made of. Brutes with a Blue Armored armor normally can be followed by a bunch of Grunts and some other normal Brutes; Brutes with a Golden Armored armor can be followed by the Blue Armored ones, and a few other Brutes; etc. While the flood it's less organized, they much more savage and attack you in large groups, still, the different forms give a lot more variety and differents functions. As it was said, this don't limit the player and give plenty of options to how to play the game.
Multiplayer:
The Multiplayer is certainly the strongest point of this game, specially after his launch. You have a good amount of modes and maps. And was one of the first, if not THE first game, to introduce a map creation system to the community and more to make the interaction between players bigger. The Theater is a really popular tool where players can create their own photos and use their criativity and the forge is a way to create not just maps but custom modes with a great variety. Unfortnatly the netcode can be tough sometimes, some shoots don't hit the enemy when they should for example.
The game is really the strongest parts of the first games. All players starts with the same weapons, the difference is the skill and the acknowledgement of the map to pick the best weapons that are in different positions in the map. Also have interesting mechanics like the Dual Wielding, where you pick two weapons of a certain type, and special Tools and Abilities like the Overshield and the Bubble Shield.
Customization is pretty simple. You unlock different parts of armor if you do certain achviements so also, depending of wich armor is, can really give pride to the player like the iconic Recon Helmet. It really gives a sensation of accomplishmentto the player.
The Skill gap is also really good. The weapons have distinct functions and advantage, and the acknowledgement of the player can really win the match if he knows where the best weapons are and if he knows how to use it.
Conclusion:
Halo 3 is a really complete game with lots of things to have and create. The campaign is really good, the story can be a little bit simple and even underwhelming but still have some highlights. Of course the best parts is the Gameplay itself that is really amazing being simple and have a lot of variety of what to do, what to use and the strategy; you can use pretty much what suits yourself the best. The Multiplayer is the part with tons with content and it nailed it. The skill and acknowledgement of the map is really important. The players also have the freedom to create their own maps with custom modes and a lot of tools to help.
In my opinion, every Xbox user should get it and give it a try, especially now that Halo MCC is on Xbox Game Pass and gonna launch on PC soon. This is one of the best games ever created and it's considered the best Halo. For me, the Positive points is just much more than the Negatives. This game is a big masterpiece, remarkable and influentional. It's Essential