Halo: Combat Evolved- 9/10
Halo 2: 7/10
Halo 3: 7/10
Halo ODST 7/10

Halo had a serious WoW factor! I've never played them multiplayer (no Live here back then), so bear that in mind:

Halo:CE
I could definitely see that this wasn't possible on the PS2. The views were spectacular, and the gameplay rock solid with good AI and challenging set pieces. I have to give it
10/10

Halo 2
Graphics were vastly improved, but the wow factor not so strong anymore. Some set pieces (at least for me) didn't work properly on Legendary and it pissed me off. The formula became slightly stale towards the end and I thought the story was kinda lame. Still, it was a first class experience, and even without the much lauded multiplayer I give it
9/10

Halo: Combat Evolved 10/10
Halo 2: 7.5/10
Halo 3: 7/10
Halo ODST 5/10

I played Halo 1 & 2 on PC and counldn't really see what the fuss was about, maybe because by the time they came out they had been outdone many times over on the PC. Also, as I don't play much multiplayer most of my opinions are based on SP.

Halo 1: 7
Halo 2: 6.7
Halo 3: 7

Halo Wars (based solely off the demo): 8

I'll probably buy Halo Wars but I'm not going to bother with Halo 3 as after Halo 1 & 2 I really don't see the point me getting it except to play the campaign in co-op when I have someone round.

http://www.youtube.com/watch?v=bXTy8VTDNjs

I've only played the first two Halos so I'm just going to base the rest on what I think.

Halo CE - 10/10
Halo 2 9/10
Halo 3 8/10
Halo Wars 8/10
Halo odst 8/10

Thats completely wrong for Halo 3. I guarantee you that every single legit 50 is skilled at the game and the majority of players could never get a 50 even if they played 3 years straight, especially in some playlists like MLG.

OT:

 Halo:CE- 9.8   No online and frame-rate issues keep it from being a perfect game. Single player was perfect.

Halo 2- 9.4  Great game but Bungie simplified some key aspects of gameplay too much resulting in a decreased skill gap. Also plagued by cheaters. Legendary difficulty was more frustrating than challenging taking away from the fun.

Halo 3- 9.6  On the verge of being perfect but some issues like the sub-par melee system and bullet refunding on xbl can cause some frustrations. The story was brilliant but you had to be very invested in the previous games to even see it. The terminals truly help flesh out the Halo universe and give the gamer a sense of how grande the war humanity is involved with really is.

Halo Wars- 8.8  The first well done and more importantly fun RTS on a console. Only thing keeping it from scoring higher is the lack of a covenant campaign and maybe a flood option in multiplayer although that debatable.

Halo 3:ODST- 9.0  Despite what some critics say this game had great value offering a great single player campaign, fire-fight mode which is sure to eat up much more of my time, a set of challenging and fun achievements that when unlocked gives the player the most coveted armor in Halo 3 known as Recon, Halo 3 multiplayer with every single map ever released as well as 3 completely new ones and finally exclusive access to the Halo:Reach beta. The only thing that made this game score less was because the campaign could have been a little longer and also I personally thought they should have taken a darker path with the story but that more opinion than flaw.  

halo 1 10/10
halo 3 9.7/10
halo wars 9.8/10
halo 3 odst 6/10
low score for odst cause no matchmaking in firefight and for me they didnt deliver $60 worth.
i havent played halo 2.

Ive only played 1 and 2 but
1: 5/10
2: 3/10(based on single player campaign only, NOT online)

Halo 2 is the weakest of the trilogy, trust me.