I play soldier just because i like to have enough ammo at all times but my friend plays medic and i feel that this is a winning combo. To be honest i feel the engineer to be somewhat useless because the mines are not a guaranteed kill and the turrets seem useless 90 percent of the time ( i havnt seen a gatling turret yet). The operative is a cool class but not for me, obviously all classes are needed to complete different objectives im just saying that i feel soldier and medic to be the best combo in terms of map control and pushing forward where as the other two seem to be neither good at offensive or defensive.
This is just my opinion so lets keep out the trolls.
The engineer buffs are a must-have. Kevlar makes it so that you only take 80% damage. AP rounds go around this, but soldiers cant afford to give every player ammo. The real kicker is the improved damage buff. Makes your guns do one third more damage. Both last until you die, you even get to keep them if you are incapped and revived. The turrets and mines are frankly just a bonus.
Soldiers are useful for ammo and crowd control, but frankly no team needs more than two.
Medics are definitely the lifeblood of a team. A team with only one is going to struggle, a team with 3 is going to push the other team hard. Four starts to push it only because the other classes start to suffer.
Operatives are an interesting class. Very helpful to the team, but only if they are good. No more than one is ever really needed unless there is a hack objective. A good operative is able to offer the team lots of useful information with homing beacon, comms hack, as well as spotting mines and enemy Ops. They also have all the tools they need to harass the enemy, disguise, caltrops, and sticky nade. Drop disguise at the right time and they can turn the tide by attacking from behind. Their control turret ability makes your team's turrets MUCH more effective, and hack turret can really rip apart the enemy if they aren't prepared for it. The turrets may be slow to react, but they hit hard when there are a bunch of people standing in front of a "friendly" one.
So, basically, the key to winning is team balance, not any one class.