Ultimately it depends on what you as the player can do with the tools you are provided with. What’s good for me may not be good for you.
For me the list looks like this:
1- Pryo: Consistent, decent, easy to play as spec, has more tolerance and is harder to go wrong with it.
My suggested tactic is stick to blue flame only, apply it liberally and diagonally to the hallways. This causes a halt in xeno advances and ultimately if you set one on fire deals a lot of damage.
Your underbarrel extinguisher is not a normal one. It has a huge capacity and much more effective in putting off flames.
So, you can combo the blue flames and the extinguisher. Flame xeno at range with blue flame, switch to extinguisher, clear a path and follow the xeno that is trying to put off blue flames so you can apply it again (or shoot it). This is how you secure kills.
2- Demo: Requires good knowledge of firing lanes, timing and movement. It requires you to have buddies near you to finish off whatever you manage to land an AP rocket on. You need good aim. Overall a good kit, straight forward to use and very useful on the front line against bigger xenos and smaller alike.
3- GL: Requires good knowledge of how a grenade flies. Also rather simple to use but counterintuitive at first.
You have a very huge variety of payloads to chose from, each of them with a different use. The M92 grenade launcher can load -any- and -every- type of grenade.
My personal favorite is the HEDP. Stay behind marines and clear every possible xeno structure with one click. Very useful when pushing.
When defending or assaulting, lob a grenade slightly behind xenos. And empty the cylinder with the stun lock.
The same stragety yields extremely better results if you use M15s and OT nades, which you should reserve for queens and higher case xenos or against a big xeno rush.
Other payloads like HIDP, AGF, etc are also very effective if you wish to block a route, or target a xeno individually for ex with AGMs, which can take big chunks of health from them. Just be smart with your ammo.
For all three of these specs, bring a strong primary weapon with you.
4- The sniper, which requires teamwork.
Basically use focus, aim and take your shot. Have a spotter light up the same target as you. This will take a huge chunk of health from the target. Once you land your shot, rush the target with your buddy with primary weapons and you can secure a kill. Not an ideal frontline specialist, but can work wonders when applied on the backlines or sidelines.
5- Scout and last for me. I have seen people pull off all kinds of tricks with scout, but to me i could not really figure out how to yield decent results from this kit, maybe just incompatible playstyle. It is quite decent for utility, such as rescuing downed marines, sneaking through or behind xenos. Ambushing xenos on the backline with mines, high-impact rounds, etc. But I never figured out what the best way to use this kit is for me.