As a very huge Halo fan, I had to comment on this. I couldn't agree with you more, HOWEVER, before I just go on record by saying that I completely agree, I want to throw a few points in the mix.
Halo 1: This was a very huge step on Xbox, really the first huge name game on Xbox and it was a brand new idea with a brand new story that we had not seen before. Those elements made Halo's campaign phenomenal because it was all brand new, not to mention the cinematic cut scenes and story was very well written and produced. Also the maps for the MP are considered classics, but they were just very well done with many different ways to gain advantages when playing with friends, etc.
Halo 2: Now before I get to talking about the MP, which I agree, was by far the best of all the Halo games, this includes ODST and Reach, let me say a few things about the campaign. If you were as big of a Halo nut like I was when this game was coming out (I literally left high school early on the day it came out so I could play all night) you will remember that when it was released, Bungie was ALREADY talking about Halo 3 and how Halo 2 would just be a linking game with the story and there would be another game to wrap up questions, plots, and other things of that nature. Now most players, with the knowledge that they would not be done with the series were already looking towards Halo 3 with much anticipation. They hadn't even finished Halo 2 and were already wondering about Halo 3. This was something that I was guilty of as well. I believe that the Devs were probably working on how to wrap things up rather than how to connect ideas as well, which caused Halo 2 to suffer in terms of Campaign. They also may have used most of their budget to create the amazing experience that is Halo 2's MP. But looking at the campaign for what it is, it still is very good. It still may be weak as compared to the others, but they wanted to leave people wanting more and with questions. The introductions of the Prophets was something that caught me off guard, but nonetheless, it was something that was unknown to most of the Halo Nation, unless you read the books, did research on the idea, etc. But by having Master Chief pursue and kill 2 of the 3, while riding off into Halo 3 on the final Prophet's ship was Bungie's way of playing the old cliffhanger card to it's fans. I honestly thought that the 2nd games campaign was good at leaving you guessing and wanting to beat the next mission to find more information on the Forerunners, The Convenant, and the Halo rings. Not to mention they added the dimension of playing as the Arbiter, which they should have kept in Halo 3, IMO. I think it got a bad wrap, as most sequels do, because the first one was so solid and there was not to be any "resolution" occurring in game.
Halo 2 MP was amazing, no further comment needed
Now Halo 3: Halo 3 I think was the cross breeding of Halo and Halo 2. The MP was very good, though it was a step back from Halo 2, it still provided enough new game variants and maps to cause for hours of mindless destruction and killing for its players. The campaign was also very good as you got a lot of questions answered and more attributes to the game were added (vehicles, weapons, enemies, etc) to make the whole experience more fun. This also called for new strategies to be made by players, who had to discover new weaknesses of enemies. Overall, I did enjoy the story of Halo 3 and loved how they set up the ending for more Halo to come. The idea that Master Chief is out there floating around and will be back for more, though most considered him dead, makes for, and excuse my language, one bitchin' game title in Halo 4. I am pumped and cannot wait to see what all they do for this title, already excited for Forerunner Weapons!!!
Halo ODST and Reach, I don't consider to be part of the Halo - Halo 3 series, though both are really good games honestly. Halo Reach answered a ton of questions that, if you pay attention to the in game conversations in the main 3 games, you would have been asking yourself. It also shows the main part of the Spartan Teams and the fall of one of the more experienced teams in the Galaxy. Answered my questions as to why Master Chief was the only one in the main Halo games. The multiplayer for both of these games is basically the same as Halo 3 IMO, however the addition of the Firefight Variant (survival mode) was a very fun was to test players grit, strategy, and teamwork. I spent way too many hours playing Firefight with friends and enjoyed every second of it.
Overall, I have played so much Halo it is almost sad. However, I encourage everyone to spend the time playing the main 3 games to be caught up for Halo 4, if you haven't already done so or are considering buying Halo 4. This is a game where you can get lost because you missed an aspect of the story in a previous game. I found myself, at times, making connections between games that I had missed in prior play throughs because I had forgotten details or conversations in prior games, etc.
CAN'T WAIT FOR HALO 4!!!