Be able to build a 29 cards agressive deck and still beat slow control decks with a single card when your main strategy fail is beyond anything else in this game history, other DK's are really strong too but hunter after get a lot of poison/lifesteal/rush beasts is the best.
This, you make an aggro deck, wear out your opponent until turn 6, then you basically switch to a control deck and torture him for the rest of the game, the card got way too powerful with new beasts every expansion
Em, i m not sure, if we talking about minions, i think, Best means broken af in vacuum, not about good in some kind of decks, or great if you have perfect curve, or if you drawn him on particular circumstances. I think, if we talking about best minion it's DK Rexxar (anytime in the game it will be broken and infinite value OP, Baku and Genn (not every deck will be broken with them, but Odd Pala who can 29 any cards plus Baku will be tier 1 deck) and Undertaker with Tunnel Trogg, ofc we talking about their time period.
But, you forgot about spells, i think, most broken (best) card in the game was - pre pre nerf Unleash the hounds, for new ones - it was 1 mana spell: give your beasts +1 attack and charge.
in their time? 5 mana Sylvanas, Fireball, Mad Scientist, 1 mana Mana Wyrm, Undertaker, the old Buzzard, 8 mana Mind Control, 8 mana Pyroblast, the old Tinkmaster and onld Nat Pagle, Mysterious Challenger, and the shaman Throgg
the most idiotically designed card is imo still Jade Idol
in a less broken or cheesy way the strongest cards are prob something like N'zoth, also Bloodreaver Gul'Dan and Shudderwock