Honestly I can't say, they all have strengths and weaknesses.

Commando: keeps enemies imobilized on the floor and have alot of armor. However, their damage is lacking and it would take a bit to kill someone.

Engineer: keeps himself healed with like every flippin skill he has, Hard especially for commandos to take down, seeing as their damage is a little lacking. However his damage once again isn't anything special.

Operative: Frikin blows the snot outta everyone with his insane damage. however, he's very weak and I could see him switching between killing or being killed everytime.

The rivalries:

Commando vs Engineer- I could see engineer triumphing because commando won't be able to damage him quickly enough. My guess it would be a long battle but since commando can't keep healing engineer would slowly finish him.

Engineer vs Operative- I could see operative winning because engineers are fairly weak and won't be able to heal enough on account of a very quick death. However, they will still take a chunk out of the operative.

Operative vs Commando- I could see both winning honestly. Commando will be trying to keep the operative on the floor, if he succeeds, his little damage+the operatives little armor would cause him to triumph. On the flip side, the operative could kite the commando then move and in and do his stuff, Due to the commandos health and armor it will still take a bit to bring him down. Just my opinions from what I can see!

~MM