Replaying the Bungie Halo games semi-recently, I came to the conclusion that:
Halo CE is alright but kinda falls apart in the later half as the difficulty outpaces the options the game generally gives you to resolve the situation, which I think is the main drive as to why Bungie tried to find a "fourth pillar" to the main gameplay elements (shooting, moving ,and melee) as a tactical element to remedy this and make there be more choices and individuality in playstytle. For its time though it was a good shooter which was playing with a new format of shooters from the old PC era shooters which isn't really better or worse imho, its more of a turn to some regard to limiting the player in regards to weapon loadouts but expanding the player's interaction with the environment as compensation to that loss, and unlike later games it amounts to more than "duck behind cover": since you have moon jump on at all times, you can easily outflank and surprise enemies with good use of the environment, use your AI teammates and create a field of overwhelming fire to suppress enemies and force them into cover, ect. Not really a move to "realism" really but more a turn to a more active battlefield and making the player less a dominating force in the battlefield but rather a facet (albeit a large one) of the larger whole, even in the final levels where most of your UNSC buddies are gone, the battle arenas have more than one faction at a time so you can play your enemies against each other. Looking back it's evident why it exploded in popularity for presenting something pretty new in an accessible format, but now it mostly seems antiquated.
Halo 2 was, as one of the devs put it, their cathedral that assembled itself out of the storm. Not really pushing the bounds in a lot of ways compared to the original but rather refining a lot of the elements of CE, making squadmates smarter and more competent in things like piloting vehicles, flanking, defending positions, ect. Granted this is partly done by scripting rather than truly intelligent AI but in the framework of what Halo 2 presents it works fairly well, the only moments when you worry about if your AI teammates are going to get killed is in situations where you feel equally worried about getting killed (that or when they dodge-roll off a cliff when they see a grenade). Of course this is also the game which introduced duel-wielding into Halo which would become the biggest weapon-related meme for the longest time after it became big via Halo, and would only be a feature in Halo 2 and Halo 3 because of the difficulties of balancing that with the gunplay. Despite all this though, its easy to feel where things they wanted to do was cut, and at the end of the day there is little else to say about Halo 2 except "Halo CE but better" and then make a meme about the cliffhanger ending.
Halo 3 was the first real attempt to build that 4th pillar that Bungie had been wanting to do but was denied from doing in Halo 2 because of the disastrous dev cycle for that game, and it came in as... the mostly underwhelming equipment system. There are neat things you can do with it, like using the turbo-lift thing to rocket yourself around the arena like you are an angel of dakka, or use the bubble shield to advance under heavy fire, but for the most part its inconsequential to gameplay. Halo 3 came with the expected improvement to both friend and foe AI and opened up the levels a lot more, since Halo 2 had more enclosed levels so the AI scripting wouldn't go mad, and even in these situations the AI can generally not fuck itself over doing something stupid. the AI uses a system of rally points to get around the map in regards to player progression part of the time and the other part of the time they will just follow the player around whenever they are accomplishing an objective. The AI can also tell when you've boned yourself somehow, like when you end up on foot during a mission where you are supposed to be in a vehicle, at which point your AI buddies will drive up in whatever multi-passenger vehicle they can find to offer you a ride. The AI also has decent threat-assessment, and they know when to sally out from a defensive position to mop up enemies and when they are overwhelmed and chose to flee into more defensible positions (the mid-part of Tsavo Highway is a good example of this, where a bunch of soldiers have a hillfort made out of traffic barriers and other stuff they have found and are under attack by a covie armored group and jetpack brigade, they retreat to their bunkers when they are getting pressed hard but when you arrive and take out large chunks of the covenant forces they sally out to help your attack). Halo 3 also loved to show off their setpiece battles in the Scarabs, which were neat at the time in having an AI vehicle with multiple modes of attack that it is using to fight off multiple parts of your own forces at the same time, and if you don't know what you are doing the scarabs can be pretty nasty, but at this point most everyone knows the trick to killing them so its more a neat concept than anything else.
Sidenote on Halo 3 ODST: Mostly just a moodier version of Halo 3 which focuses more on survival in a hostile environment without aid than big setpiece battles, but it can be pretty fun if you like both jazz and halo 3.
Halo Reach was the sendoff to Bungie's halo as they parted ways with Microsoft, and its pretty much what you'd expect from such a sendoff, it didn't focus on innovation as much as Halo 3 did but worked on polishing up what was already there to give the experience people knew in the best way they could. They did take another crack at the 4th pillar thing with Reach as well, now with their new equipment system which was less useless by virtue of the fact that the equipment wasn't single use and included useful things like a sprint which not only increased movespeed but modified your jumping angles and heights so you could play with terrain more, jetpacks to zip from place to place and rain down lead from above, stealth cloaks for if you want to dick around and ambush your enemies, and things like the lockdown ability which made you immune to explosives and vehicles running you over at the cost of making you immobile. While it wasn't quite as ubiquitous as any of the 3 existing pillars, they were all situationally useful and did a lot more to vary things than Halo 3 did. The campaign took on some of the slightly darker thematics laid out in Halo ODST for their main campaign, setting up the game as the losing battle to save the planet reach (the UNSC's biggest planet besides Earth) from the largest covenant force ever encountered. The plot also acts as a good ramp for the game's difficulty, so new players can still smoothly get into the game at the early levels and be prepared for the large difficulty spike towards the late game. Levels became a bit more linear from Halo 3, but that was likely more due to the fact that the battles were denser with more enemies and allies than ever before in larger spaces, with more vehicles, air support, and troop movements between locations on the map as the battles unfold. I didn't really miss the more freeform levels as much, I tend to find the more concentrated linear experiences where most of the creativity comes out of how you interact with the things around you are more fun than the games where its just a bunch of stuff lying around in a sandbox for you to interact with from place to place. Halo 2 and Halo Reach probably stand for having the biggest difficulty spikes in Halo though, and that is probably why they both stand as my personal favorites.