If you're good at it, SnD helps a lot. Otherwise, I find Drop Zone to be good for shooting down the air support and going in the drop zone for 20 Exp. every second.
you level up to level 80
the best game mode for ranking up in mw3 is probably domination because it is pretty long. if u do really good u could probly level up or get close to leveling up each game.
In MW3 you level up your gun by using it. You get xp by getting kills or destroying enemy equipment like a helicopter. If you use it enough you will unlock new attachments camouflages and other things.
Level 80.... 20 prestiges
The easiest way to level up is to feed Pokemon Rare candies.
completing puzzles are the best way to level up.
There is no best way, just change monsters as you level up. edit: this guy is a dick thanks mister know it all
Train it.
By playing.. ;)
You level up your secondary that you want attachments on.
In the special ops, you just level up your level to like 25, and if you spend more cash, they get riot shields :D.
For me the best way i have found to level is by doing all the quest in each area before you move on to the next area.