Good at:
-Running away
-Losing games
-Feeding kills to the enemy
-Abandoning the team
-Dying

Bad at:
-Capturing objectives
-Defending objectives
-Supporting the team
-Getting kills
-Staying alive

F tier

Good at:

-Reviving heavies
-Healing heavies
-Movement support for heavies
-Dealing damage from behind heavy shields
-Chasing enemies that try to run away from heavy

Bad at:

-Capturing objectives
-Defending objectives
-Dealing with lights

A tier

Good at:
-Capturing objectives
-Defending objectives
-1shotting idiots playing light
-Getting kills
-Staying alive
-Supporting the team
-Winning games

Bad at:
-Chasing enemies
-Reviving teammates

S++ tier

LLL - absolutely unplayable garbage comp F- tier
LLM - unplayable garbage comp F tier
LLH - garbage comp but maybe heavy can carry F+ tier

MMM - could be viable with constant healing and reviving C tier
MML - same as above but worse D tier
MMH - good setup with lots of movement and support and one central tank A tier

HHH - viable setup with tons of shields, destruction and healthpool but lacking support and movement A tier
HHM - perfect comp with one movement, healing and reviving support and two big boys to assault or lock down the objective S tier
HHL - viable if light runs away preventing a team wipe, otherwise worse than medium in every way C tier

HML - viable if heavy and medium work together but dragged down by a light class doing nothing B tier