If I were to rank the operators from my experience in overall benefit to the team. This is not taking weapon quality into account, just gear
Attackers
Essential:
Thatcher, a cure all of ability problems
Top Tier:
Thermite, getting past reinforcements is generally key to pincering defenders
Ash, best option for getting through walls without getting feet shot out
Montagne, walking straight into a room and just laughing at the defenders while your teammates pick them off. Really really helps getting into certain rooms.
Middle Tier:
Fuze, clusters are super good, but not essential and not as useful to the rest of your team
Twitch, drone is good for going in an picking off annoyances, or for getting a defender to move
Sledge, near infinite use is nice, but ash is generally better
Blitz, His instant flash is great, but he doesnt have the speed/survivability of montagne to be as useful to the team
Glaz, great at flushing enemies out or picking them off...when the map allows
Fucking awful please buff tier
IQ
Defenders
Essential Tier:
Mute, need your best chance to stop thermite
Bandit, also good at stopping thermite, and possily even better than mute. A good listener can pop electric on the wall right as he hears thermite putting his shit down. Hurting thermite and breaking the charge. Loses out over mute when it comes to defending castle walls
Rook, nothing easier for benefitting the whole team than buffing their armor
Top Tier:
Jager, stopping fuze clusters can be good, but he doesnt get every one. Stopping grenades and thatcher EMPs tends to be better. Thatcher still laughs from the other side of a wall
Castle, his walls are amazing, but certain maps you are often better off just not having them in the way, which is the only reason I don't have him in essential tier
Middle Tier:
Tachanka, placing his gun is a pain in the ass and hurts him. He does a good job at laying some brutal cover fire on the right map, as well as preventing entry.
Smoke, His grenades are usually only good for buying time, which can be enough.
Pulse, killing repelling fuzes that are trying to plant is fun
Doc, so many people die too quick to ever actually revive, especially since headshots are instant kill. Still picking up a teammate for 75 health can be powerful, and if you down safe or trade downs its always nice to get yourself back up.
Kapkan, most people in ranked will never fall for his shit, but its still good in doorways that force your enemy to give away their position and deal with the trap. And you still get some lucky kills on shield guys and enemies you manage to bait.