Short answer: As of late, I am trying to focus on getting better on perks that orient around headshot bonuses or extending zed time, using assault rifles, single shot rifles or pistols so essentially Commando, Gunslinger, Sharpshooter.. eeeh.. SWAT? Maybe sometimes, for the armor handicap (getting armor at wave 1). Used to play Support. Tight Spread is great. Good perk for new player, objective friendly, team friendly, cause we all want that armor in wave 1.
Long answer: Our team composition used to be Support, Gunslinger/SS, Berserker/Demo. We were terrible, still terrible. Friend who was Gunslinger/SS was better than me, loved reload cancelling. I frequently went Field Medic to support him on boss rounds - back when Hans was the only boss. The only Medic XP we ever got was from manually healing each other or using Medic pistols tbh. Other friend who played Berserker/Demo - same skill level as me. Eventually, reload cancelling got fixed and the "Gunslinger", as I call him. he went Survivalist/Medic with a touch of Demo. Other guy stayed with his perks, never really got better, doesn't want to try a new perk :(. Mainly play with the "Gunslinger" friend. We both are terrible now. Sure we definitely made progress but we're bad or rusty. We play The Descent, Powercore and Nightmare on HoE and truly suffer - unless there's a new Objective or Weekly, in which sure, we're trying to beat 3p HoE Steam Fortress.
I'd love to be good at a Berserker but it's hard.