Your opinion on the top 3 list

My top 3:

Teleport -  This card is pretty hard to argue in my opinion.  It's a key component in one of the most infamous deck in the current metagame.  It's probably the card most often complained about for being overpowered and in need of a nerf.  It breaks the basic structure of the game, Zombies aren't allowed to play zombies after Plants, Teleport changes this.  

Mixed Up Gravedigger - This card is often gamebreaking.  It removes the ability of plants to set up proper blocking.  It may shift zombies away from in-place plants.  He's 5/5 for 5 brains, while not the most efficient stats available they are certainly close to the best, and coupled with his ability he's likely undercosted.  He's a synergistic of course, and it's important to consider how widely available the cards he synergizes with are.  Since those cards are any zombie they should comprise a large part of every deck, no problem there.  If you own him and he's available to your hero would you hesitate to play him or his he an auto-include? I think he's an auto-include, that makes him among the best.

Grapes of Wrath - This card is perhaps undervalued.  It's a huge body with a massive damage death effect.  It's very hard to play around, you can't ignore it because of it's 6 power, you can't kill it because of the 6 damage it deals.  The best you can do is bounce its death damage off your shield.  That can be hard to setup though, since the Zombie player is actively trying to prevent.  This card doesn't suffer from the drawbacks of many other powerful cards, it has solid stats, doesn't care too much if it's blocked or not,, etc.