Assault rifle offers the most versatility, the attachment lets you put a grip on any rifle which several don't let you otherwise. And the alice though the G36 is pretty subpar lets you use a assault rifle with the second level of a suppressor on the terrorist side if you're really into being quiet but still want to use a assault rifle instead of a SMG. Should be at least the secondary choice of most people just for the foregrip.
The SMG specialization also improves your experience using that class of weapons quite a bit with it's attachment, it lets you use the less damage and range killing Suppressor 1 on several of the weapons that only let you use the 2nd level for whatever reason. The skinned p90 on the commando side is also appealing to many though I prefer the MP7 over it myself.
The sniper specialization is only worth taking as a primary and only if you want to use variable scopes on a suppressed rifle on the mercenary side. If you are the type who only ever snipes with the thermal scope there is no point in taking it since you can already stick that on any rifle.
Pistol...well the heavily shitted on pistols in this game need even the meager stat boost you get from the specialization so it's always a good choice.
For shotguns I probably wouldn't ever pick it as one of my specializations, the grip it adds doesn't really seem to help at all and the USAS 12 is horrible outside of point blank range so the weapon isn't too appealing unless you're the kind of scrub who only knows how to play by charging and spraying right at people. Now if the specialization weapon was the M490 instead I would jump at the chance to get that weapon on the mercenary side.
The LMGs are just about worthless, if they were at least a little consistent the super scope the specilization gives would be fun to snipe with but you just can't rely on the guns to hit anything when you need them to. If you are just screwing around it is sort of fun to stick that scope on an ameli but that's about it.
