on 5cp:
2x Natascha Heavy (hoping to see a rebalance in tomorrow's update because natascha is broken. The damage resist combined with a quick-fix or vaccinator makes you a raid boss)
2x Medic (Double quick-fix probably, but as long as you have 1 quick-fix you can switch the other one to something else. 1 kritz would be good because it would counter the quick-fix of the other team)
2x Whatever
You can decide the remaining classes probably.
Are they also running heavies? Sniper
Rollouts? Soldier(s) with Disciplinary Action. Any of the 3 banners + 2 Natascha heavies is probably ridiculous as well
Demomen would be a pick that's never bad. Fast rollout to compensate for the heavies being later to mid. Good counter to Vaccinator if the other team wants to counter the Heavies with Bullet Resist.
I'd probably go with 1 Banner Soldier with Disciplinary Action + 1 Demoman. Scout is obviously also great but too much hitscan and the Vaccinator will start countering your team
I tried this once with some friends and I'd never had so little fun in my entire time of playing TF2
edit: Also works fine on koth. Never tested it on gorge but you can probably just change 1 or 2 "whatever"s for engineers