i thought this would be somtihng fun to talk about so here we go:
1) Dark Armned Dragon took a whole format for itself. DAD return and tele-dad could easily bring this card out a reck you. If you play a dark deck you play dark armned dragon. Today it stands as one of the msot powerful card ever and still is till this day.
2)Judgment Dragon like dark armned dragon this card was a boss monster and get this.. it is still at 2! JD is jsut so easy to summon and has the advantage of not being a dead card late game unliek DAD.
3) Rescue Cat with x-sabers and thier new support this card just get's more broke. The way it works along with monk is even more broke. You know a card is powerful when the name of the deck starts with a card that is limited to one.
4) Destiny hero malicious this card was a key card in tele-dad decks since it allowed to bring out level 8 synchros with easiness. Withh d-draw and stratos this card was broken and would give you quite the advantage.
5) plaguespreader zombie this card is one card that makes zombies what it is.. you recycle this card like crazy. With mezuki and all of the other zombies you got yourself a synchro fest.
6) Gorz the emissary of Darkness single handily can beat you opponent. you get a free 2700 beat stick with a token!
7) Stardust Dragon while it doesn't come out a much as the days of tele-dad stardust can be quite pain to get of a field.
8) Brionic this card would otk you with zombies. Also could be a good out to stardust dragon. Somtimes it could let you push for that last bit of damage you have to put in.
9) Gladiator Beast Gyzarus- pop 2 tag out for more pluses?
10) Light and Darkness Dragon this card is quite the beast. While lightsworns may beat it pretty easy against other decks it can wreck you. You have to use up cards just to lower it's attack. The dandy and drill warrior combo makes this card easy to summon.
hmm alot of these cards are old cards but we know they are powerful since they are still used today.