I think there are three camps that make up the 300 million people who have seen the Avatar films:
Those who love them and are so into them that they will rush to ride the stupid NA'VI River Journey ride at Disney World
Those who want to see the new technology and special effect tricks James Cameron is using that could leave a impact on movie making
Those who feel they have invested so much time in the first movies that they have no choice but to keep going and finish them
I fall firmly into categories two and three. I liked the first Avatar and the special effects used. I even show parts of this one to my history class as a perfect example of exploration and imperialism. But it is not one of my favorite movies. I saw the second movie because I felt I had to. I knew everyone would be talking about it one way or another and I didn't want to be left out. And sure enough, that movie was fine. I saw it once in a theater in 3D and have not thought about it since. The only things I remembered from it were that there was a kid that looked like Cha-Ka from Land of the Lost and that around one hour of the movie looked like a national Geographic documentary for an ecosystem that doesn't exist. There was so many scenes of CGI characters and animals swimming around. It was like going to a fake aquarium. But it looked cool so I didn't feel I wasted any money.
Who else remembers Cha-Ka form Land of the Lost in the 1970s?
So when Avatar: Fire and Ash was released, I resolved myself to the fact that I would have to see it. Not only that, I'd have to see it in Lie-Max in 3D. Luckily my son was in the exact same boat so at least I'd have someone to commiserate with during the nearly 200 minutes of runtime (That is 3 hours and 15 minutes).
And then something unexpected happened... I really liked it. Yeah it is basically the fanciest cartoon ever with about 90% of it being computer animated... but I really liked it. In addition, it is basically the exact same movie as the first one... but I really liked it. There are some really silly parts that required a serious suspension of disbelief... but I really liked it. It was a direct ripoff of Lord of the Rings, Star Wars, The Bible, and just about every sci-fi or fantasy movie I have ever seen... but I really liked it. And believe me, no one is more surprised than I am. I keep writing "I really liked it" to fight against my instinct to say "it was fine". Because that is what I have grown to expect from these Avatar movies.
Fire and Ash is the best of the series. How can this be even though it is so derivative? It basically improved on everything from the first movies. This one just did everything better. The 3D didn't make me sick. There was no wasted National Geographic shots (they saved that for the credits). There is no dragging love story or significant time spent on lore. So what did they fill the three hours and fifteen minutes with? ACTION!!!!! Lots of action. So much action in fact that I never got up once to use the bathroom. the time flew by. And that is saying a lot for me because I have a bladder the size of a thimble. I was memorized the entire time. In addition, the Villain is absolutely incredible! Varang looks so damn cool and she is so freaking ruthless. She also has one of the coolest powers in the series. To be honest I think I'd rather see prequels about her story than the next one in this series.
I also liked Cha-Ka... I mean Spider a lot more in this one. That was incredibly helpful as he is basically the only human actor in the entire movie. There are a few soldiers and businessmen, but they are only onscreen for maybe 15 minutes total. Spider actually had an interesting story and Jack Champion shows that he actually learned how to act since the last movie. He is also the main MacGuffin in the film so having some sympathy for him also helped.
TLDR; This is the best Avatar film because it has the most action and least love story, national Geographic like scenes and Varang the villain kicks ass!
One thing before you leave. For any Lord of the Rings fans out there, do you remember the scene where Pippin and Merry show the Ents what is happening in Middle Earth and Treebeard says "The Ents are going to war"? If you loved that part of the story, you will truly enjoy Avatar: Fire and Ash.