I'm gonna do something different and review the game by comparing it to Halo 2.

*Campaign:
Off the bat I would say overall this game is analytically better than halo 2, most of my gripes have no factual bearing and just gripes that are not necessarily "bad".

Gameplay: While the movement physics are sound as usual, I think masterchief moves wayyy to slow, this is ussually noticable on levels that have no vehicles. I have no idea why they changed it from halo 2 where he moved at a very nice pace. The grenade throwing also seems to be more sluggish and I find it to be harder to aim grendades more than all the other halos.

Sandbox:
Guns:
The sandbox is for the most part better than halos 2s, but with some exceptions, the AR is utter trash compared to the other ARs in previous and following games. The damage output is hilariously low and is worse at suppressing enemy fire than in CE, this is issue only gets worse when considering there is 4 automatic weapons that are all at short to medium range (those being AR, SMG, Plasma Rifle, and Brite Spiker) leading to a cluttered part of the sandbox, it also doesnt help that the AR is the worst out of all of these since all the other guns can be dual wielded. Also why is there BOTH the AR and SMG in one game, there is a reason why bungie removed the AR in halo 2. This is because the guns share almost the same exact role in the sandbox. Its evident that the only time these two guns appear in a halo game is halo 5 where both weapons are extremely different than past incarnations. The flamethrower is also a pretty bad weapon since if you hold down the weapon you end of burning yourself leading to a hilarious death. It being an extremely close range weapon that also lowers your movement speed even more doesnt help either. To be more positive, the BR was perfected in campaign since its weaker, and has slower projectiles

Vehicles, Grenades, and Equipment:
The new flame and spike grenade are also very welcome, tho you only see the flame grenade only twice or thrice and the spiker is kinda just a worse plasma grenade, but its ability to stick on walls and its quick detanations keep it fresh
Vehicles are kinda an iffy in this game, the new mongose is AWESOME but sadly not very useful outside of its insane speed, but its still cool, the brute chopper is the best vehicle, its like a more powerful rustic ghost and i love it. However the prowler is very boring and just literally a brute warthog, its lazy and no one likes it. The hornet is cool on its surface, it being the first UNSC flying vehicle, but the fact that the passengers get no turrets to fire makes it useful only for a solo pilot. Now the part which is absolutely terrible is grunt controlled ghosts, since you can now pop the helium tanks of the back of the grunts, its almost impossible killing a ghosts driver unless you are good at using a sniper, they also output insane damage on heroic and legendary
Begining with halo 3, the devs have been trying to add a fourth pillar to halo (first 3 are Guns, Melee, and Grenades) for Reach and Halo 4 they added armor abilities and for Halo 5 its spartan abilities. Halo 3s attempt was equipment. Equipment was rly hit and miss, the most important and best equipment is bubble shield, this gadget allows you to protect yourself from enemy fire, but you cant shoot back and others can step inside and take it from you. Its a great sandbox tool. However the others dont have the same amount as quality, the others range from useless to situational. Drop wall is very useless since it doesnt even protect all parts of you and flare just causes your screen to go white........ im not kidding. Other things like trip mine are very situational and hardly used. This concept is structually sound, and has lots of room to grow

Overall the sandbox of halo 3 for campaign is pretty solid tho I wouldnt exactly say its considerably better or worse than halo 2s sandbox.

Story: While the other two are debatable, no one can argue that halo 3s story is better than halo 2s story for 2 reasons, pacing and characters. The pacing for halo 3s story is bad, you just loiter around earth, until the game decides its time to go to ark. The characters in this game are also considerable worse, for starters, Prophet Truths voice actor has been changed for some reason. Halo 2 truths voice was maniacle, cunning, and two faced while this truth is just an insane yoda sounding lunatic that for some reason wants to light the ring????? In halo 2, truth and the other prophets came up with their religion as a means to brainwash all the other species in serving them. He never wanted to commit galaxy suicide, he just wanted to control the ring. Lord Hood and Miranda keyes for some reason also get less screen time and is only used as the commanders for crows nest and pretty much nothing more. I kinda wish they also gave Arbiter some more screen time since its just chief finishing the fight, its also him, but I like the touch of chief letting arbiter kill truth, that was pretty nice.
The biggest sin of Halo 3 tho, is the need to kill off as much characters as you can, like I get killing truth, since he is the villan, but did they really have to kill miranda keyes, sergeant johnson and guilty spark. Miranda keyes death was so stupid, she basically just crash landed in the control room to save johnson, but she didnt bring any men, and didnt use the fucking turrets the the pelicans have, its so stupid. Then 343 guilty sparks decides to kill johnson because he wants to control the ring? His motives are so stupid, they make no sense. His job is to kill the flood on the ark, and that what they DID. Its understandable in halo ce, since chief decided to not kill the flood and backstabbed him. Then as if that wasnt enough guilty spark shoots a lazer at sargent Johnson and he dies. His death just feels rushed and unnecessary. Overall this one is ok, and a huge disappointment since it wasn't as good as halo 2. The ending scenes were pretty good tho, and it was a nice resolute ending.

Levels: Now I can say that the levels in halo 3 for the most part where better than halo 2s. The pacing of the levels were pretty good (with two exceptions) and each level felt long enough to be satisfactory, while not too long to overstay their welcome like with halo CE. The two exceptions are cortana and floodgate, floodgate is a good level but its just too short. The level cortana however is so bad, it is easily the worst level in halo HISTORY. Its the sole reason why I have only completed halo 3s campaign from start to finish once. Its flaws are many and would take too long to explain but TLDR the level is too long, made even longer since the layout is so confusing and stale. The only thing you see is purple metal and flesh colored flood. The only enemies you fight are the flood and there is no vehicle section there. It has everything halos worst levels have mixed into one disgusting level. Cortanas and graveminds speeches are ANNOYING AS HELL since they stop you too a halt everytime they speak, its annoying in every other level when cortana does this but this level has this more than usual :(

*Multiplayer
Just to keep it short, the maps are just worse, the only original iconic maps I can think of in this game is the pit guardian, and valhalla. Compare this to halo 2s Ascension, Lockout, Zanzibar, Ivory tower, Midship, and Shrine, and Headlong. All previous critiques to the movement apply here. The sandbox in the multiplayer is terrible here, halo 3 has this issue with netcode, but tldr, this allows some of your bullets to just not register, this makes guns even worse since most halo 3 guns have slower projectile speed than before. The BR is very finicky to use and it leads you to over lead your shots. Previous issue of BR dominating the game still is present in halo 3 but the netcode and weaker guns just make the guns rly poor. One positive I forgot to mention is the needler is no longer dual wieldable so you don't get than one kid in your lobbies any more.

Overall this game is awesome easily in my top 3 halo, I just wish it had more polish and fine tuning.