You ever just take the time to look at all the other spartan’s in your lobby? their favorite weapon always being the same unless you see that one rare individual? Lucky mines the Assault Rifle and all its glorious splendor 
Mine’s the Grav Hammer. 
I like the assault rifle and the battle rifle.
Covenant Carbine
Carbine.
/suprise
Okay you people rock XD and respect for that Grav hammer
Assault Rifle and Carbine
DMR is my most used weapon though, because of Big Team and SWAT.
I think the OP was asking which weapon is listed on your Service Record as your favorite weapon, not which you think is your favorite.
Love the Beam Rifle. It’s size is obnoxious as hell.
Snapshotting people is great way to get your score up and ordinance drop fast as hell.
Needler…Gotta love that Pink Mist
What is actually listed as my fav on my stats? Wait for it…
The DMR
I love that glorious feeling when I scroll through the sea of people in the lobby who all use the DMR most, and then I look at my own glorious record which proudly says “Battle Rifle”.
F*** yeah.
Mind you, big respect to Carbine users. You guys are pretty badass.
Is it bad that I have a standard UNSC weapon AR? Because i love to death these promethean weapons and just want to feel unique by having one of them as my service record weapon.
carbine
It says mine is the BR but that is because of campaign and spops, I am almost mastered the AR because that is my true favorite.
The Energy Sword, a weapon made for true warriors. But for the starting weapons, the Carbine, I love high ROF.
I find an appreciation for each weapon as work through the commendations.
Right now I’m using AR (only UNSC loadout weapon remaining) and Boltshot (cuz the plasma pistol commendation is gonna be a giant pain.
Fave loadout weapon: DMR/AR depending on map
Fave ordinance weapon: SAW
I started with the DMR exclusively just cause of Reach, but the Battle Rifle is my favorite now. Definitely in my every loadout, plus the storm rifle suits me and I’m all about precision weapons, weird huh?
Battle Rifle, no matter how much I lose because I refuse to use a DNR…
And I made it so my main loadout is now the BR and AR combo. Woo!
Suppressor is my most used weapon. I like rapid fire guns.
Conversly, my personal fav is the Rocket Launcher, but it is far too scarce in Halo 4. Rockets only games are the best. Whatever happened to Scuds?
In my service record its the DMR, mostly because of campaign I think, it proved invaluable against crawlers and watchers.
My actual “favorite” weapon to use is the Light Rifle.