S tier:
Sadar - only spec that can reliably kill the queen. If some kind of god was playing it, they would clear out any and all T3s. I swear half my deaths as crusher are from sadar. Just them holding a tube up is enough to stop xeno pushes. In high pop they are a great contributor, and in low pop they can determine a match. Even a bad sadar can get a T3 or 2 at the cost of gib’d allies. out of ALL the specs, if sadar was the missing spec in a single round, there would be an uproar.
A tier:
Grenade GLer - the potential to juggle stunlock xenos is high. They cannot stun every single xeno like sadar, but those stuns on defender/warrior/any other T1/2 that decides to frontline is brutal. Even if the xeno escapes, they will think twice before attacking, making them another effective paper tiger.
Pyro - They do well to lock down the front. They can also chase in flames and get kills when the xenos retreat and resist. The counterplay of ravs, queen, and long range acid, along with cases of FF and warrior lunges stops pyro from S tier. That and most xenos do not get hit with bullets as they fight pyro (marines stop shooting around pyro, pyro himself doesnt fire), so often times the xeno escapes in the frontline battles.
AMR sniper - Just barely in A tier, they do two things well: deny T3s AND the queen from getting a proper attack in, and securing those low health kills. Even a novice can go pew pew pew and just soften up the xeno frontline. With a team of sweaty spec players, I would expect the sniper to get the least kills, but their impact to the battle flow is constant and relentless.
Scout - They can hive dive to free caps and kill core. They can kill almost any xeno alone, including a rav, in the xenos backline. They can simply frontline and still kill that rav. When xenos fight in open areas, they can assassinate the almost dead and healing xenos. Probably the best boiler killer of all marine options: I bet scout could trade his life in most cases for a boiler kill. They would be S tier but they require the most knowledge and skill to reach their peak. A crap scout can be just another MK1 AP user in terms of battlefield effectiveness. Knowing positioning, what they can get away with, burst fire only, 1st shot incend chambered, bayo for godly melee, pre-meds… I wonder myself what tricks the veteran scouts are withholding.
B tier:
normal sniper - the tricks that the sniper can do COULD make it better than AMR, but most often than not they are not effectively used. The flak slow is painful, but there might be only one time I have ever attributed flak as the reason I died. Maybe not even one. The incends are good but often used as another annoyance tactic. The normal shots do less damage than AMR. Honestly I just willingly accept the shots from sniper, especially when they try to spam multiple incends and flaks when I am safe. On paper they can use these abilities to secure kills, but in reality it just doesnt work that way.
C tier:
dart GLer - Their impact to the game turns out to be similar to the annoyance of sniper, without the kill secure capabilities. Any xeno worth their salt will see a dart went in them and will retreat while it explodes. Landmines are often either purposely early triggered, seen and avoided, or triggered with marines nearby for FF. Cool concept, but EVERY other spec does well to secure those kills, and the darts just don’t even come close.
edit: seems like the only thing we can reliably agree on is normal sniper and SHARP are mid