Oh man, if we're talking about OP stuff. Here's a list.
You were probably around for IL-2/T-34 ram combo. It wasn't OP per say but it was a high risk, high reward for a big swing in either direction. Either an enemy heavy tank died or you were out 250 muni and a T-34. Heavies were pretty powerful but they all had the weakness of dying to this combo.
Now going farther back. When Rifle Commander was released, the E8 was an insanely good tank. It was super accurate on the move and the AoE on the shell shredded infantry. You could run circles around enemy squads and still blast them with every E8 shot. Really fun to play until the E8 ran into a mine.
You used to be able to do call-ins on enemy home bases. So you would see artillery or loiters on your base sometimes if you were losing particularly hard. The worst abuser was the OKW tactical nuke from OKW Salvage Doctrine (iirc). Back in the day OKW had lower munitions and fuel income but their OST allies could build caches to up their income. OKW players using this would save up about 600+ munitions then call in the arty barrage on their base (most of the time on USF) and destroy their entire base with one click. It was pretty nuts. You can actually see it here: https://www.youtube.com/watch?v=7Rp6gleSyDw
Penals used to get flamers (as someone mentioned) which was amazing on them because of the high member count and the already high AI damage. HOWEVER flamers back in the day had a random chance of exploding and killing units if being shot at.
Volk shrek blobs used to be a mainstay of the OKW army. They always had 3 volk squads with shrek upgrades which would counter any vehicle that allies could throw at them. What also sucked is back in the day, calliope wasn't in the game so only Soviets had rocket arty to counter these shrek blobs.Volk shrek blobs were insanely durable but also bled MP like crazy for the OKW player.
USF Riflemen used to have smoke grenade so they could self smoke MGs. What usually happened though was 2 Riflemen squads would hit an MG, one to smoke, and one to push up and throw grenade. Although 1 squad could do it themselves. I think MG42/34 suppression was upped so that 1 squad couldn't do it alone.
Up until recently, Soviet 120mm was basically a broken unit that didn't work for its price. The barrage took longer to fire than the autofire and the AoE was non-existent. Then it got fixed and then nerfed.
OST Close Air Support doctrine used to be insanely good. It had a button that let you instantly convert MP (then eventually fuel) to munitions, no pickup required. It had a anti-infantry strafing run call-in and an anti-armour strafe call-in. No one ever used the anti-infantry strafe because the anti-armour strafe destroyed infantry better than it damaged tanks. So you would have OST players just constantly calling in anti-armour strafes on infantry.
I'm trying to think of other stuff that was broken that come to mind but I think most of the stuff was mentioned like Tiger Ace and ISU-152 HE shells.