Does it count that I've been playing the game since the Channel/Fireball combo was legal? Back when being "exiled" meant there was no way back into the game, and Swords to Plowshares did that rather efficiently. I've seen people lose because someone played The Rack back to back with The Black Vise.
Back then, there was no "Commander" format. It was basically a free-for-all, and people didn't understand the purpose of proper deck building. I had beaten almost everyone I had played with a 45 card deck, and the aforementioned combo wasn't in it. A friend of mine was adamant on using a 150 card deck that didn't have enough land to draw, and then kept wondering why he lost all the time.
One of the cards that you just don't see anymore was an asswhooping in disguise. Howl From Beyond. If you had a creature attack, and go unblocked, with enough black mana, you'd play that for 1, then tap all the black mana you had. Each one gave the creature +1/+0. In the end, that card ended many a game.
My best deck however, is a toss up. Either it's the deck that only has 3 creatures in it, and relies on direct damage in the form of Fanning the Flames (a fireball with a buyback), and hand manipulation (opponents hand, mind you) with things like Capsize, which would put any creature back into the owner's hand, and it had a buyback, so it could be used again, and again, over and over. It also had Reins of Power, which took all the creatures from one opponent, put them all on your side for one turn, and gave them haste. Or the mono black deck centered around Megrim, which causes any opponent that discards a card to lose a life for the number of cards they discarded. With 3 of those in the deck, there was a good chance of drawing it early, or more than once. Then it had creatures that, if they hit, you'd discard, and discard spells. Often I'd win without having ever attacked.
Best combo though has to be Wrath of God + Armageddon + Dingus Egg + Reverse Polarity. Granted, Wrath of God doesn't matter in that combo, but it's fun to watch people's reactions when you wipe the table clear of the creatures they spent so long to cast. The Dingus Egg caused 1 damage for every land that was buried. Armageddon destroys all land. Reverse Polarity (an Interrupt) reverses the damaging effect of any artifact, so instead of losing the life from the Dingus Egg, you'd gain that many life.
I'm also a DCI judge, and often have to judge local events.