It is also a viable option to equip the heavy with more armor due to its lack of mobility.
Heavy: For heavies, players will find the most success if they emphasize increasing grenade usage (Opportunistic) and healing from the damage they do (Bloodlust). The Utility Belt mod on boots will reduce cooldowns on frag and stem grenades, which can work well with the Boom Grenade skill.
Here are some general tips for how to gear up your characters based on their class: