I think you say it best 

Halo: CE - Really fun game. The single player is good, but it doesn't finish as good as it starts. Of course, it suffers from the same problems all the Halos do. The levels drag on for a little too long, many corridors and rooms look very similar*, and the parts with the Flood are generally not as fun. The multiplayer is just great. The one problem is the pistol is really over-powered, especially in the more open levels.

Halo 2 - Best of the series, and my favorite FPS that isn't on the N64. Everything good about the first Halo was refined to near perfection in the second. I can't think of a level that I didn't really like, something I can't say for the other two games. Multiplayer is even better than before with the re-balanced weapons.

Halo 3 - A disappointing entry in the series. The new gameplay additions and new weapons didn't do anything for me. The level design for a few of the later levels is just bad. Multiplayer is still as great as it ever was, but since I don't like the new weapons, I don't like it as much.

*Not a problem with all levels, but there are times where you go through a corridor that looks just like the corridor before it, only to enter a room that looks just like the room before it.

EDIT: The_God_of_War's post reminded me about the Arbiter.  That's another reason why Halo 2 is my favorite.

I completely disagree on what you said about Halo 2.

I like Halo 2 but it's no where near on the same level as Halo CE and Halo 3 in terms of SP and MP IMO.

Halo CE: well what can i say this is the game that started it all and revolutionized the FPS Genre. 9.5/10

Halo 2: PERFECTED online multiplayer and IMO had a very good campaign.
9.8/10
Halo 3: Added upon the things presented in the first 2. Good multiplayer and awesome 4 player Co-op. 9/10

Halo CE - Great campaign; addictive offline multiplayer - 9.8
Halo 2 - "only" good campaign (still great in co-op); better multiplayer due to Xbox Live - 9.2
Halo 3 - better campaign than the 2nd game, but not as good as the 1st; awesome multiplayer - 9.6

There is something wrong with that video. There are plenty of times in the first Halo where I know I 'missed' the head shot with the pistol, but the guy died anyway.

Halo: Combat Evolved- the greatest FPS ever and one of the greatest games of all time. Has the best SP of the three because of more open ended environments (landing on the Halo for the first time and exploring it on the Warthogs was amazing), a ton of ways to approach situations/firefights, awesome vehicles and weapons, co-op extends the replayability (I must have finished it at least 4-5 times), very solid AI, some pretty funny dialogue and is just purely epic. The Library level gets tedious but other than that I loved every second.

Halo 2- Couldn't quite live up to the first, it's more linear and I didn't like how less of the game was set on a Halo, a chunk of it took place on Earth. But playing as the Arbiter was cool, especially with the cloak ability. Overall the SP was a lot of fun but I didn't experience the MP so I can't comment on that. Brutes aren't as awesome as Elites and that fucking cliffhanger, ugh. It's a 9/10 game but as I said, doesn't quite live up to Combat Evolved.

Halo 3- Better than 2, not as great as CE. My complaints are similar- not as many open environments (although more than there were in 2) and aside from the final level none of it is set on a Halo. It's short too so overall, the SP disappointed me a bit. With that said it's still the best FPS campaign this gen. The gravity hammer and new items were great additions, as well as the absolutely epic Scarab battles. The final boss was disappointing (basically a floating ball) but when do you ever get good boss battles anymore? As for the MP, it's the best I've ever played. A ton of different modes, the stat tracking, Forge, etc.

Halo Wars- It's pretty straightforward stuff, and this is coming from someone who barely plays RTS's. But it's acessible, easy to get into and of course, fun. Just like all Halo games. I played the campaign through 3 times and the online can be great if you're playing with friends. (Anytime I play it with strangers, the pricks either quit or give up) Overall it's one of the most underrated games of '09.

Halo - Perfect in every way. Except for The Library, I loved this game.

Halo 2 - Hype killed it. More time would have been welcome but the fanboys had to have it.

Halo 3 - Great game. Never beat it though since I don't have a 360. I watched the story online and it was excellent. Not as good as the first game though.

How you feel about Halo 2 is how I feel about Halo 3.  The hype part, I mean.

1-Good 

2-Excellent

3-Decent.