Engineer, same as most of the other people who've replied. Nothing like being close to a VSAT and spotting some glowing red stuff to EMP!
Nothing better then raping the person who is camping with shock charges because you have tac mask.
Tac mask gotta love seeing the flash bangs and concussion grenades fly from around the corner letting your know they are coming lets you know to stop and take aim or even toss out c4. Can't leave out running through shock charges and cleaning out a room. Engineer is good too I just prefer tac mask.
I think Engineer is the best tier 3 perk. It lets you see all the explosives lying around, even the C4 when its being thrown.
Tac Mask is the 2nd best.
When I run around a lot and run into head on confrontations, dexterity is basically necessary. I have it on every class I think except my sniper one, because I don't try to go out and quick scope or anything all the time lol Even climbing fast is useful. Since so often you're dying by people sprinting at you and suddenly pulling up their gun, you need it to help out unless you're mostly posted up somewhere already
But almost every perk in that tier is worth a slot, depending on the situation of course. Extreme conditioning for things like CTF, even Search. Tactical Mask and Engineer are good for everything. Awareness helps in Search now as well (at least in league play lol) Only one I don't really use is Dead Silence, since most don't use Awareness. I might use it sometimes though
i only use engineer and dexterity. tac mask and dead silence are okay but still i rarely use them except for maybe hc or deathmatch. extreme conditioning is nice with ballistic knife/knife only set up but thats about it. awareness i have no idea how useful it is.
Dexterity is my first choice because throwing emp grenades in front of me means engineer is pointless to me, getting into buildings by climbing still catches alot of people offguard.
Tac mask is my the other i use alot, if someone in the lobby is using stuns/flash this goes straight on