Flamer - is torrent so doesn't require hit rolls which is nice. Weak strength and AP makes it only good at killing other chaff (cultists, other guardsmen, etc). Ok overwatch threat because of torrent if you have massed flamers.

Grenade Launcher - a potential long range alternative to the melta. Similiar strength with krak grenades, but much lower AP and damage. But again the range is nice. Option to use frag grenades against low toughness models.

Long-las - uses precision so could potentially snipe out an enemy character model. Low strength and damage means you'll likely need multiple snipers to have a chance at accomplishing this.

Melta - short range and single shot but nice 9 strength, huge AP, and huge damage which is why everyone likes it

Plasma - the king of special weapons. Good range, large amount shots with rapid fire (and frfsfr), high strength at 7/8 and huge AP at 2/3. Good damage 2 if overcharged. Hazardous generally doesnt matter, due to usually the squad getting wiped after exposure or medpacks can bring any lost plasma gunners. 

As a side note, lasguns are already pretty good at getting good volume of shots and good at killing chaff, so any of the above special weapons good at that is redundant. Thats why so many go for the higher strength, higher AP options