ISA

M82 - G Assault rifle.

What I liked about it: Good accuracy, good damage rate, low recoil and fast reload.

What I didn't like about it: Slow rate of fire, low ammunition capacity, no shotgun or Grande launcher attachment.

Helghast

StA - 3 LMG

What I liked about it: Monstrous rate of fire, high damage rate, reloads fast.

What I didn't like about it: Very strong recoil, not very accurate while on the move, "you can't equip either a primary or secondary badge! Unless, you pick it up off somebody you kill." - Thanks Meptron for adding another fact about this weapon.

Grenades + Sentry
It may be cheap, but oh boy is it fun.

Mine is the default assault rifle that the ISA uses. 

I'm in the minority in liking the helghast assault rifle. although it has lower accuracy and higher recoil, it has a large clip and higher rate of fire, meaning it kills quicker at close and medium range. Although its sights aren't quite as good as the ISA rifle, it can still be good at a distance if you practice with it, and tap the trigger to avoid excessive recoil. Also, the helghast rifle is better for firing from the hip and since most people use the ISA rifle, your opponent usually can't use your ammo if he kills you.

edit:
@Mordukai: you forgot one disadvatage of the LMG. Using it means you can't equip either a primary or secondary badge! Unless, you pick it up off somebody you kill.

I use the M82 almost all the time, love that gun.

I like how the Sta-3 can be one of the most accurate guns in the game if you crouch single shot it.

But my favorite is the SMG. I can get headshots all day when I get in range.

default assault rifle

"@Meptron: Thanks man. I edited in, I hope you don't mind that I used your quote. "

M4 revolver.... it is definatly worth the time to get it...  other than that.. i use any gun .. i like taking other peoples guns... but ill drop any gun if i come across the sta -3 awesome gun.. 

mine is the assault rife.

isa assault rifle for big distances and this one: