PC user here but I've played the hell out of this game so my thoughts:
Guns:
I'm an AR guy all the way, so its the Famas/Aug until you unlock Commando (lvl 44), then it dominates. If I'm in a sniping mood I'll also use the bolt action rifle (the others are pretty horrible). I haven't tried the AK74u grip and rip mentioned above though, so I may try that later.
Secondary:
Big Maps - the lock-on anti-air launcher with 2 shots.
Smaller Maps / Domination - Ballistic Knife. At first I wondered why anyone used this thing when you already have a knife but WOW it is crazy at the moment. On Nuketown I just run around hacking away in the houses and it's near unstoppable (granted, you become "that guy"

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Grenades/Equipment:
Do whatever, just know that the claymore is pretty crazily powerful.
Perks:
The two that are pretty non-negotiable imo are Hardline Pro and Slight of Hand Pro. Both easy to get and level to pro. For Perk 3, I prefer Hacker Pro, but Marathon Pro works too.
I adjust these slightly depending on game type, so my "Search & Destroy" class has Ghost, my sniper class has the Scope one, and my capture the flag class has Marathon.
Killstreaks:
The most powerful KS at the moment is Chopper Gunner, which will dominate if you do 2 things: as Hacker Pro, before activating, scan the map for red outlines of SAM Turrets (will shoot you down fast) and kill them or wait till they go down, and make sure and focus down all enemies standing still first, as they are probably trying to lock on to your chopper. I've gone around 80-5 in some lucky rounds with this KS.
Then assuming you are Hardline Pro, Care Package is pretty much a must, as the second chance really helps you get something useful.
For the last one, it really doesn't matter - I normally go with the masses and do the RC-XD, but they are kinda cheese and I'd be fine if they were taken out of the game.