I personally think Maniacal Laugh is the best card in the game (don't tell anyone). I have at least three of them in every deck that I can. So obviously, that would be my first choice for you.
While Zombot 1000 is great, he is very expensive, and then easily blocked (unless used with Teleport of course).
Octo-Zombie is cheaper, has rampage (best trait in the game), is amphibious, and you get two for the price of one!
As far as Zombot vs. Cornucopia; I have four of each, I use them both frequently, and only once ever have I seen a Zombot played immediately after a Cornucopia.
It's a great idea, and it seems like it is meant to be, but realistically it isn't going to happen often.
