Quick Recovery decreases the time it takes for you to wake up stunned teammates. Since opponents usually don't use non-lethal weapons, and you might be far away from that teammate even if they did, it's not a very useful skill.
Monomania is a universally helpful skill, displaying tactical data on enemies you hit, allowing you to see them for a few seconds after they duck behind cover, and letting your teammates see them too. At level three, the display time is increased and it still only takes up one slot.
Sixth Sense puts glowing markers around traps, allowing you to avoid getting caught, and higher levels increase the detection distance without taking up any extra skill slots.
NARC is another useful skill, showing enemies that have you locked-on. Only at its highest level is it really useful, showing enemies that are merely targeting you. Before that, the enemy has to be using lock-on, and at level one you actually have to take damage. In any case, you have to be in the enemy's sights before the skill kicks in, so it's usually too late to do anything about it unless you already saw them anyway. And because leveling up a skill requires using it heavily, the only way to get Narc to level three is to spend hours in a "training" game, a match set up by a host with the specific intention of everyone mutually using certain skills and not actually fighting.
Scanner is another useless skill, as it requires you to grab an enemy, inject a scanning plug into him, and then let him go. If he happens to stay alive long enough for the scanner to give you any useful information it might be useful, but you're really just better off fragging the guy when you have him in the first place.