At this time i got 3 level 20 characters; a heavy soldier / engineer, a heavy soldier / medic and a medium engineer / operative.
The heavies are armed with a Chinzor (muzzle break) as primary and a Kross (muzzle break, speed sling, adjusted sights, drum magazine) or Gerund (COGA, front grip) as backup.
The medium is armed with a Gerund or Carb-9 (muzzle break, red dot, front grip, drum) as primary and a Kross as backup.
I found out that although i have great fun while playing as a medium that i am mostly play as a heavy and have a blast while doing so.
As a heavy i try to find the chokepoints on primary objectives / routes and clear them of enemies so my team mates can perform the objective, or do them myself.
With the heavy you have less mobility so it is key that you try to keep the fire lanes as thight as possible and use terrain to block of parts of the battlefield.
If you can do this the amount of firepower you can put out with the Chinzor / Kross combo is very gratifying, and can keep the enemies off objectives for a serious time.
For this role i find the soldier class best, as you can resupply your ammo and have acces to the grenades, which really helps.
If needed i can switch to my secundairy class to accomplish objectives or help the team in another way.
When playing as medium engineer / operative i am more mobile, changing class to fit objectives and battle conditions.
This is more a support role then an offensive one, but can benifit the team greatly by gaining objectives, capturing command posts and criticall support.
I haven't gotten the hang of the light body type yet, but now i'm levelling a fourth character as a light (only level 8 so just started).
I find the reduced weapon options / firepower of the light tricky as i tend to gravitate to the objectives, where there are lots of enemies and i tend to get overpowered.
To be a good light i will need to adjust my style, but am still learning.
But overall i just enjoy playing the game