SM
Walk softly carry a big gun.
To Me my brothers.
For the Emperor.
Clense the unclean.

IG
I will crush you with the tanks of the imperial gaurd.
For the Imperium.

    Actually the Ork one you're refering to is "'Ere we go"  short for
'Here we go' - unless of course you mean that annoying "WAAAAAAGGGGHHH"
crap that they have the munchkins scream out at games day.  

As for the Necrons, their official battle cry is "CHA-CHING$!$"  :)
(And the official RGMW Whirlwind sound effect is "Ha-Wang!" {which
should be screamed in the face of any Goober that shouts 'Waaagh'} in
honor of our own John Hwang.)

Honestly, I'm not really certain that there are 'battle cries' for all
the various armies per say.  I don't really see the Necrons for example
having a battle cry.  What would it be?  "Rust never sleeps!"???  What
you're talking about relates more to an army's fluff/background.  Even
the Chaos one you've got listed could be replaced with "For Horus and
Humanity" if you were doing an 'old school' Black Legion chaos army.  
The Imperial Marines would have a virtual ton of them as there are a
boat load of different well known chapters (even if they don't all have
their own codex) with unique historys, etc.  

Tau - "For The Greater Good!" comes to mind, but that's about it off the
top of my head.

    Myrmidon

The Index Astartes articles list the battle cries and/or sayings of
individual marine chapters (loyal and traitor) but I can't remember any of
them off the top of my head.

"For the Emperor, For Gav Thorpe" - IG
"Gaaaaaaav" - Orks
"Suffer not the Gav to live" - SM
"Cheese for the Cheese God" - CSM
"For the Greater Smell" - Tau
"CommarThorpe" - DE  also the refrain from a Boy George song "Comma
Comma Comma Comma Thorpe, he cheeses and writes, he writes of cheese..."
"We want his Baby" - SoB
"" - Necrons, refering to the C-tan Fromage Bringer.
"Ssssssk!"  - Nids, it translates into, Feed the great Cheesemeister.

Mar - actual answers are overrated, cheesey sort of speak - IO

Khorne: "Blood for the Blood God"

“A dagger in the dark is worth a thousand swords at dawn.” – Warhammer 40k

“Blessed is the mind to small for doubt.” – Warhammer 40K

“Innocence proves nothing.” – Warhammer 40K

“There will be no rest this side of the grave for all those who seek
perfection.” - Warhammer 40K

“A soldier obeys without question, a commander commands without doubt.”
– Warhammer, 40K

“When cities burn and armies turn, and flee in disarray,
Cowards will say ‘tis best to fly and fight another day,
But the faithful know it in the marrow when the die and fall,
It is better to have fought and lost, than not to have fought at all.” –
Warhammer 40K

“A soldier’s faith in his commander is his strongest weapon.” –
Warhammer 40K

[snip]

Thanks!

LONG LIVE THE SQUATS!