I've been trying to do a lot of experimenting with the class combinations to find some good ones. Here's some I've come up recently.
'Rushing Recon'
Primary: VSS
Secondary: M93R
Gadget: C4
Spec 1: Red Dot/Ammo Upgrade
Spec 2: Magnum Ammo
Spec 3: [your choice]
Uses/Best squad formation: This is best used with two teammates, Medic and Assault, following behind you. The Assault person gives you ammo and the Medic heals/revives anyone when needed. Really good for Rush: Attackers(This has not been performed/tested, and I would like to if anyone is up for it.).
'Suicide Bomber'
Primary: Shotgun(any)
Secondary: MP 412 'Rex'
Gadget: C4
Spec 1: Explosive Upgrade
Spec 2: Improved Demolitions
Spec 3: 'V Elect' (Electronic Warfare Package)
Uses/Best Squad Formation: This is best done alone to minimize casualties. Best use on a Quad Bike for maximum speed and effectiveness(If it is done right you may not die). If you rush in and damage the station and you are able to find cover then it is best that a squadmate with the Medic class spawned off of you to help hold down the location. 'Suicide Bombing' could also be a perfect distraction to the team. All most everyone on the enemy team will be rushing over to either A or B (whichever you bombed) so if the other station was up other teammates could then rush over to the other station and arm it, giving the team stuck in between which station to disarm, and will likely go after both at once in small groups. It would still be advisable that around the same time as the suicide bomber rushes in that some (1-2 minimum) teammates with the Recon class could get into position at the alternate station, taking out any stragglers making it easier for the teammates to move in.
Simpler combinations that don't take as much timing and strategizing:
'Unstoppable Medic'
Primary: MG3/M60 LMG
Secondary: -Your Choice-
Gadget: lol I think everyone knows this.....
Spec 1: MedK H+
Spec 2: Body Armor
Spec 3: -Your Choice-
Uses/Best Squad Formation: This is perfect for the medic that wants to help as a major support unit. The body armor helps in damage resistance and the Mad Kit healing + helps heal teammates at a faster rate. The reason I said MG3/M60 LMG is because the MG3 has the highest firing rate, but the M60 LMG has the highest damage and (I think) has an ammo drum of 200 so it has a large capacity for less reloading, and more ammo.
'Massacre'ing Light Tank'
Description: I saw a video by penguinz0 (check out his channel) on his 'Fun Tactics' series about using the Light AA tank (I don't know the proper name at the moment. It's the tank that is seen through out the map Port Valdez. It's the one with the big mounted gun on the top with a grenade launcher for the driver). You just need a class that uses the V WHD (Vehicle Warhead). You are easily capable of taking down multiple enemies and could be used stationary, or with a driver. It almost 'overpowers' the Light Tank because it greatly increases the attack power/damage to take out just about anything easily and quickly.
If you have any good class combinations then just post it here.
NOTE: I have not actually tried/used any of these tactics except for the Suicide Bomber and the Rushing Recon. I do understand that most people probably know this and it's sometimes common sense but newer members that see this may not.