I think tac mask, engineer or dexterity are the must use 3rd tier perks.
Extreme conditiong isn't that great because rushing isn't as popular because you die so fast if you get caught with a couple bullets, which is really weird because it seems like from my perspective it seems like i shoot someone 4 or 5 times before they die. But sometimes I get caught with 1 bullet and die it seems, but then when it shows the kill cam it shows they shot me more than once, however when I was actually playing it never showed I was being shot because screen didnt turn red.
Awareness/dead silence doesn't really help me anyways because I am hearing impaired, so that's why when I play online and use my mic I normally request team mates to talk loudly. This is also why I have been trying to spread word to treyarch to create a hearing impaired dlc or settings thing for zombies and multiplayer. With the bad hit detection if my screen doesnt turn red sometimes I don't know I'm being shot at.
Tac mask is great for avoiding tactical stuff/ combined with flak jacket this is a good combo for running into buildings to kill campers/ people playing defensively.
Engineer is a must have if the enemy is getting a lot of scorestreak, combine Engineer with the black hat and take out their scorestreaks quickly. Another good thing about the blackhat is when you destroy enemy equipment they give you a good amount of points. Enemy equipment gets you 50 points each and scorestreaks scores you 150 points each. So I recommend carrying 2 black hats paired with engineer in one of your classes.
Scavenger is an awesome perk if your running without a secondary... some people say why do you like scavenger, well it's simple scavenger refills not only your ammo but also your lethal and tactical equipment, which is great if your posting up in an area because you can just keep throwing down shock charges and bouncing betties or see someone coming and hit them with a flash and a semtex.
This post is a bit lengthy, but this is just some of my insight on some of the perks. i like other combinations as well but I think this is long enough so I don't go too off topic