I personally loved the Portal From Mk2. It was such a mysterious stage, especially with the ability to fight smoke if you uppercut the opponent and pressed start when forden said "toasty!".
Its really cool how MKteam has brought it back in full 3d. I wish they also did that with the living forest since it has so much potential for death traps and such. But hey, right now im happy with the portal and I hope they let us play all the retro stages in the arcade.
What is your favorite stage so far that we have seen?