Vayu, Malicious, and Mezuki

They're all similar cards. Each of them has decks built around them because they give you free cards.

Vayu. You can remove it along with Sirocco, Elphin, Armed Wing, or Armor Master to get a free, effect-less Blackwing Synchro Monster. Easily abused with Burial from a Different Dimension, can be searched by Black Whirlwind, and can be dumped by Armageddon Knight or Dark Grepher. (Unlimited.)

Malicious. You can remove it to get the other copy from your deck. Can be searched with Stratos and dumped with Armageddon Knight or Dark Grepher. It's also a Destiny Hero, which gives it a mess load of support. (Semi-Limited.)

Mezuki. You can remove it to revive any Zombie in your Graveyard. Can be searched with Goblin Zombie, which it can revive for more free cards. (Semi-Limited.)

Vayu is a nice piece of support for its theme. It's not extremely broken, but it's good. Malicious is somewhat better, and definitely more splashable. However, it can normally only be used once.

Mezuki is just broken at 2. If it were Limited, then it would most likely only be used twice per game, three times at the most. At 2, it can be abused like crazy. It can get you a Tuner (Plaguespreader Zombie), which neither Vayu nor Malicious can do. It can also get you Goblin Zombie, which is basically the Zombie version of Sangan.

Not to mention its splashability. People can throw two of them, a Goblin Zombie or two, and Plaguespreader Zombie into a deck and make it work, much like how people use the Destiny Draw engine. What's worse is that it gets more free cards.

Mezuki revive Goblin Zombie (+1). Tribute Goblin Zombie for Caius and search Mezuki (+1). Proceed to get the Mezuki into the Graveyard somehow for another +2. Not even the Destiny Draw engine is that bad.

I'm tempted to say that I'd rather face a Judgment Dragon than Mezuki. Both are probably going to 1 with the next ban list, anyway. I mean, they Limited Black Rose Dragon while they Semi-Limited Mezuki and left Judgment Dragon at 2. Who does that?