Halo has three supreme advantages. The first is that it has a compelling expanded universe which can be expanded to incorporate new concepts. The second is that innovation is at the core of its nature. The third is that it places a high value on balance and variation.

In other words thanks to its universe nothing is forbidden. You can have any weapon, or any tool as long as it is technological in nature. Want a item that makes you jump real high you can have that. Want to run up walls you can do that. Want to have a bomb that uses gravity to lock your opponent in place you can do that. How about a rapid fire mortar with shells that create a wall of fire sure you can even have that. Imagination is the only limit.

Every game in the series seeks to innovate. Every installment in the series sees new weapons, new tools, new modes, new vehicles, and new features. Put simple the series is only going to have more interesting facets as it goes through time. It shouldn't stagnate.

Finally it is all about building on previous balance, and expanding upon it. Bizarre as it may seem Halo is one of the few first person shooters where there is no utterly useless weapons. Granted that you have the skill to use them properly. Every weapon has its trade offs to contend with. As long as they keep the balance and variation going it should suit just about every play style.

In the end Halo can go just about anywhere it wants, and given all the originality each game brings I really want to see what they will come up with next.

I've only played Halo series... but i wish play all of them

Halo(Well I played from 1-3 and still had a blast)
Killzone(1 didn't cut out for me)
COD( I only liked 4)

Although I wouldn't want to see anymore Halo for now. Or if there will be give it a break first. KZ well I played it very much but KZ2 is a blast.

When you say halo what do you mean?

Halo the original?

2?

or 3?

I hated 2 with a passion.

3 is overhyped imo and way overloved.

As for CoD i hate ALL COD games, so with that said  Halo (No matter which title) >CoD.

As for Killzone 2, never really bought it due to i try to keep the over-the-top violence games OUT of my gaming list this gen, games dont need to be bloody as hell to be good, but still if it wasn't i'd say it would be awesome for the graphics and oodles of epic motion blur alone!

So in Conclusion...

None of the Above... But there is one Epic FPS this gen that wipes out all the rest...

Halo series hands down. Call of Duty is just milked shit in its current state - the only thing i enjoey in this gen CoD games was natzi zombies mode. Killzone series had so far one decent game that got borring later in the singleplayer campaign and two bad ones so i don't know.
i say:
Halo series
Perfect Dark series
.
..
...
Killzone series
Call of Duty series

Out of these three? Killzone > Halo >>>>>>> CoD

It does 

Firstly, Killzone 2 is hardly violent. You liked God of War, yes? God of War makes KZ2 look like a game where you blow flower petals across a field set to quiet acoustic music (does such a game exist? It would probably be awesome).

Secondly, I want to think you're joking when you say TimeShift is good, but then you like Untold Legends and Armored Core For Answer, so I don't know.

AC: For Answer is better than 80% of other PS3 games, Kantor.

According to Metacriticlol, there are 381 games on PS3. 20% would be roughly 76. I have no problem accepting that AC4A is the 77th best game on PS3, so we're in agreement. Metacritic, however, has it at #268. Hmm.