One mana for an artifact that taps for two. Every deck would play it, and the games where someone has it on turn one aren't close.

Channel turns life into mana, and 19 life is a lot of mana. A deck with it can win on turn two off one huge creature or one burn spell, and there isn't much the other player can do about it.

For two mana, each player is cut down to whoever has the fewest creatures, lands and cards in hand. Decks built around it make sure that's them, so in practice it wipes out one player and leaves the other alone.

Flash puts a creature into play at instant speed for two mana, and it dies unless you pay most of its cost. Pair it with a creature that wants to die, like Protean Hulk, and that's a two-card combo that wins at instant speed.

Equip for one, and when the creature dies you draw two. Any deck with small creatures draws cards faster than the other side can keep up with, and it never runs out.

It makes X hasty 2/2 knights and makes you the monarch if any of them connect. It's good at every point in the game, and the extra card each turn means whoever cast it usually stays ahead.

X mana, and the opponent discards X cards at random. It only needs one black mana, so every black deck in the league ended up playing it, and losing your whole hand on turn four isn't much of a game.

Haste and menace for three mana, and when it attacks you can sacrifice a creature or artifact to deal damage to anything. It moved up from the watchlist as more red decks picked it up over the last four leagues. No other three-drop ends games as fast.

Every turn Oko turns one of your creatures or artifacts into a 3/3 Elk with no abilities, so the cards you built your deck around stop doing anything. Its loyalty goes up while it does this, and few decks at this budget can remove it. Games with an early Oko go long, and one player isn't really playing.

Nadu gives its owner a card or a free land whenever one of their creatures gets targeted, up to twice a turn per creature. Players target their own creatures with free abilities like a zero-cost equip, and with a few creatures out that draws most of the deck in one turn. It's too easy to set up for how often it wins, and the turn takes forever.