The most underrated perk in my opinion is toughness because players i rarely see people use it. It really comes in handy when using a AR or sniper rifle. When there is some lag it wont affect you that much when your shooting.
Underrated scorestreak is the sentry gun. In an objective game like domination and demolition a sentry gun placed in a good spot can keep the enemy from advancing forward. Forcing them to take a different route. It can also be controlled.
Underrated tactical is the trophy system. It's really useful when capturing objective. Mainly in domination when capturing the B flag, all of a sudden you see like 3 or 4 explosives coming your way.
Lethals are all very useful.
People don't like it because they think running and gunning is the "only" way to play (lol), and the Death Machine, understandably, does NOT let you run and gun with it while using it (you can "sprint", but only slightly faster than people walk, lol), and taking it out and putting it away take quite a long time compared to normal weapons (and then, when taking it out, another second before you can fire it).
I'd have to disagree with the Sentry Gun being useful, unless you are playing less then average players who don't know what a EMP grenade is. One EMP grenade can take out a Sentry Gun. Thats just devastating. EMP grenades probably need to be nerfed in this game.
My must use perk is Scavenger. Picking up extra lethal and tactical equipment is a huge help, as well as extra ammo. Theres no point to carrying a secondary weapon with this perk on.
I also find Blind Eye to be helpful as well. You don't have to worry about being hit by a HKD (unless beside a teammate, in the open) or a stealth chopper. And I find you are only hit by at worst, half the lightning strikes that get called by enemies.
I would say any score streak that reveals enemy positions is the most under rated. Everyone likes mass carnage but most people duck inside if they can when those are coming. The revealing ones work even if they duck inside.
Most under rated perk in my opinion is probably the mask. I think its helped me more than a few times.
Perk: Tactical Mask. With all the Flashbangs, Concussions, and Shock Charges thrown around all the time, the Tactical Mask GREATLY shortens the time you're affected by them, so when people come charging in, hoping for an easy kill while you're stunned, you're already over it and kill them, lol.
Scorestreak: Dragonfire. While some may say Sentry Gun or Guardian, I wouldn't say they're "underrated" as much as seen as "currently pointless" due to EMP Grenade usage (which can get refilled with Scavenger). I rarely ever see the Dragonfire used outside of Care Packages. One mistake I commonly see when it does appear is people just let it sit pretty idle over areas and just try to shoot people from above rather maneuvering it around while shooting.
Lethal: Combat Axe. It actually requires good aiming and compensation for arcing and such with a narrow hit radius. People don't like this, especially with moving targets. They'd rather go for stuff with a big radius that they can toss/set and forget about it because they have far greater chances of kills.
Tactical: Black Hat (Sensor Grenade is pretty close, then Trophy System). Probably because, outside of using it on your own Care Packages from a distance or on enemy Betties, Claymores, and Shock Charges (in which case, the enemy gets a WARNING that they were hacked. =\ Might as well have the announcer go, "LOOK OUT! ENEMY IS NEAR YOU!!), it's either "useless" or takes way too long TO use in terms of using it on Scorestreaks, leaving you extremely vulnerable while using it.