Here's my thoughts:
--If your a really low level (below 20) play on domination. Use the Scar w/ silencer (beastly gun for starting off), then as a secondary gun have the spas-12 shotgun. For equipment use semtex and either flash or stun grenades (semtex take a little while to get used to, but are 100x better once you get used to them). As your first perk you want sleight of hand, stopping power (hardline is shit, all it does it give you a killistreak one kill earlier, but then you die easier than you would with stopping power since it gives you more than one kill, and get kills faster) and steady aim (depends on your style, steady aim is for those not comfortable with knifing, but if you are then commando is alright too).
--For those above 20, but below level 48 play on Domination, HQ or if you think you can do get the kills fast enough, TDM. You should either use the Tar-21 w/ silencer or the RPD w/ silencer (RPD's are amazing, but you better have quick reaction if you use it though since they suck close up so you have to be able to knife, and the tar even w/ a silencer has good range and is really strong). Secondary gun I like to pick the PP200 w/ silencer (my favorite pistol by far). Your equipment should be the same as I said before (semtex w/ either stun or flash). Your first perk should be scavenger PRO (pro is important since it starts you off with a big clip, and you don't want to run out of ammo when building a streak). 2nd perk should be Cold- Blooded if you have it yet, otherwise pick stopping power (being hidden off their radar is HUGE in building a streak if you aren't very experienced). 3rd perk should be ninja PRO (cold-blooded takes you off their radar so you don't need scrambler, and ill it does anyway is give away your position to enemies. Ninja PRO silences your footsteps too so they can't even hear you, or see you on heartbeat sensors, and since you have a silencer on your guns, they can't find you at all unless you have a dumbass teammate next to you spraying bullets everywhere).
--For all people level 48-69 you can pretty much play whatever to try and get a nuke except search, cage match, and even on team tac I guess. As your first gun you want the ACR w/silencer (extremely accurate, and racks up kills). And everything else stays the same (semtex, stun or flash grenades, scavenger PRO, cold-blooded and ninja PRO)
--For those at level 70 you probably already knew all this stuff. You'd want the AK-47 w/ silencer and everything stays the same as before.
--killstreak setup--always end with the nuke, but doesn't really matter before that for the more highly skilled people. If you are new I would suggest harriers, then the AC130 (not the chopper gunner since people always shoot it down now and infinity ward dropped how much damage the chopper gunner does anyway).
For people wanting to get it without this setup people call "noobish" you can try the attack helicopters and then the pavelow (but beware, pavelows don't count towards killstreaks unless you haven't died in the match yet).
Other Tips:
--if you haven't yet, change your sensitivity to maybe at the lowest 4. This helps so you can turn and fire onto enemies faster. (mine is 7)
--You can change your button layout to tactical so circle is knife and the R3 stick is crouch. This helps so you can dropshot easier, and it can make knifing easier (especially for me since the analog stick are worn out and don't always respond when I push them down ;p)
--When going for a nuke, contrary to what you may think, it's not best to camp in one spot. What works the best is when you constantly keep moving to different cover since people will probably be looking for you since you just killed them.
--I noticed how a lot of people like to duck under cover (which is fine) but then pop back up in the same spot and just spray bullets at me. That is stupid though, since I would have been aiming at where you where the whole time, and then have to find me when you pop up. If you are getting fired at, GO AROUND BEHIND THEM. most times you can slip back and then you can just go around a building or do whatever, and then they are laying there waiting for you to pop up so you can just knife them, tbag or whatever you want.
--BTW if you are completely out of ammo with both guns, grenades and flash/stuns, and someone is firing at you, advance towards them behind cover, then wait for them to be forced to reload and run at them and knife them. Not only do you get a challenge for it, but then because of scavenger you get more ammo.
--When you want to build a streak but are camping, ammo is hard to come by, so now is a time you want to not be spraying bullets (stupid thing to do normally anyway), and you should only fire if you know you can kill them before they get away since you'd be giving away your position too.
edit:
--I forgot to mention that you don't want to look down your sight 24/7 since it narrows your vision, and creates what I call "tunnel vision" (where you are only aware of a certain are, not the whole place), and also it causes your guy to turn slower when you are zoomed in, so it helps to look around then look down your sights, and then when you find someone to aim and fire at them faster.