Theorycraft
Four slots, four jobs: someone who is shot at, someone who fills the Advantage pool, someone who stops an Injury becoming a death, and someone who answers cover. Almost every strong squad is a different way of covering those four, and the two compositions the developers warn against are the two that skip one.
Reasoning, not measurement
The class tier list asks whether a Specialization is worth a quarter of the squad. That is the right question for one bay and the wrong one for four, because the second-best class at a job you have already covered is worth less than the fourth-best class at a job you have not. A squad of four excellent attackers is a worse squad than three good ones and a Scout.
Four is the standard limit, though EA say certain missions let you control additional characters — plan as four and treat the extras as a bonus. The roster runs to twenty, and Bonds are what make a wide bench expensive: relationships build between a settled pair, so an Operator you recruit and rarely field earns you nothing.
The four jobs
Several classes can fill each one, which is exactly why this is a composition question and not a ranking.
Decide who takes the return fire, rather than letting the enemy decide. The Heavy's taunt rides on its primary attack, so choosing the target costs no separate action.
Advantage is earned one point at a time by damaging enemies, caps at ten, and starts every fight empty. Every Specialization has an Ultimate priced in it.
An Operator reduced to zero health takes an Injury, and three Injuries mean permanent death on standard difficulty. Healing a wound in a standard Medbay bed costs a campaign cycle.
Cover reduces chance to hit and some of it is destructible. Displacement removes the enemy from it instead, which no hit-chance roll can refuse.
Named by the developers
Not our inference. Both are stated in official material or developer interviews.
Worked examples
Each of these is the same four jobs solved for a different campaign. None has been tested.
Best for Permadeath-safe · Survivability · Sustain and healing
Core synergy
Combat Stim multi-shot scaling
An untested first-campaign squad, built only from confirmed mechanics — the reasoning is ours, and it has not yet been played in the retail build.
Best for Permadeath-safe · Survivability · Sustain and healing
Core synergy
A dedicated Advantage generator
A squad assembled for the hardest of the four difficulty settings, built around the two things a harder fight actually takes away: Action Points and margin for a missed shot.
Best for Permadeath-safe · Survivability · Sustain and healing
Core synergy
A dedicated Advantage generator
A squad for the ironman setting, where you cannot reload past a bad battle — built to lose slowly rather than to win fast.
Best for Permadeath-safe · Sustain and healing · Anti-droid
Core synergy
Combat Stim multi-shot scaling
Developers specifically recommend Ion weapons against Separatist droids — build the squad around that counter.