I guess if you're really into Halo's fiction you would be bothered by Bungie dropping the ball so badly, but calling Reach the worst? Halo 1 literally copy/ pasted hallways over and over again, Halo 2 is pretty inconsistent and clearly unpolished (framerate drops, crappy textures, the ending), Halo 3, I have to say something about.
I think Halo 3 is really not that damn great. I don't like Halo's vehicles that much. The reason Halo's vehicles are successful is because they control so well and add variety, right? Halo 3 says "screw that," and throws nearly entire levels that are just pure vehicle section. Driving a Warthog in circles while the AI shoots at everything and you just hope you don't get hit. Forget about Ghosts, you'll be shot right out of it, and good luck killing Wraiths. If you try to grab a Chopper don't even think about coming within 50 feet of your enemies, because they'll easily overwhelm you. Also it can't strafe, so try not to get rammed by kamikaze Brutes in their own Choppers. And when do you get any actual cool vehicles, that let you really tear things up? You get a Scorpion and a Gauss Warthog once each, both in the same level and in the same section, and I think that's it.
Aside from that, do I even need to do elaborate what is so horrible about the levels Cortana and Halo? Last two levels of the trilogy and they're both crap.
The regular fights are pretty much as good as ever, but Halo 3 just DOESN'T LET YOU have those fights. You have to get into vehicles and drive past hordes of Covenant and do all this stupid shit with Wraiths and Choppers.
I will argue this to the death!
But really, Halo 3 has the most dynamic vehicle sections.
We could start with Tsavo Highway where the only vehicles are the Chopper, the Warthog, and the Wraith. It's a good combination to get the player introduced to vehicular combat and for the more advanced player to experiment with the sandbox. When using the Chopper you have the choice of cannoning the Covenant from afar or going for the much riskier splatter. You can also boost at the correct time on rocks to send you soaring. There you can either bail out or stay in the vehicle for another pass around. The Warthog in these encounters is obviously much more fun with co-op partners, but it works with the AI as well. A beginner can learn paths and AI line of sight, and advanced players can go for the splatters or get in the passenger with a power weapon. Wraiths are only available at the end of the mission, but they provide for an interesting mortar section at the end.
The Ark is where the vehicles really start to shine. When you get your first vehicles in the level it will either be a Mongoose or a Prowler if you managed to kill just the occupants of the vehicle. Then the next immediate encounter, you have the chance to hijack either a Ghost or a Chopper. And right after that there is an abandoned Warthog. You now have almost the entire light ground vehicle armament at your disposal. My personal favorite is to get Sniper/Rockets on the back of a Mongoose and let the AI drive me around. The best part is that if I die, 9/10 I'm not angry about the death being cheap. There is an entire optional encounter that is ridiculously fun to take as a Mongoose passenger.
Right after that you get some Scorpions dropped off for you. The AI even tells you that "tank beats everything" and it is a hell of a lot of fun to blast through with the tank. After you clear the building and exit the other side you enter one of the most dynamic sections in any Halo campaign. You have the choice of every ground vehicle, including the newly introduced Gauss Warthog. At the end of the push, you have to fight a Scarab. And guess what, you still have the choice of every vehicle.
The Covenant also lets you tackle vehicular encounters with incredible variety. How will you face the dual Scarab encounter? In a hijacked Covenant vehicle? In a tank? Gauss hog? Regular hog? Mongoose? Hornet? Some are definitely more effective, but all are viable.
Here's the best part about all of it though. There's only two situations in Halo 3 where you are forced in a vehicle, and they aren't any of them that I mentioned above. It's the flying section on The Covenant and the last mission of the game. See
The Pedestrian. It's definitely a good watch.
To quote a specific part of your post to clear something up: "And when do you get any actual cool vehicles, that let you really tear things up? You get a Scorpion and a Gauss Warthog once each, both in the same level and in the same section, and I think that's it."
You seem to expect vehicles to be a god machine. That's not the case by any means, and if you are looking at them like that, then you are quite simply doing it wrong.