What are some typical popular decks, and metagame decks out there? And cards that you guys feel are a MUST play in Grand Melee?

Martell house card.

why Martell? And what kinda martell deck?

I don't personally run Martell decks, but several friends do, and a lot of the draw (and correct me if I'm wrong here guys) comes from both the ability to give and take icons at will, as well as the fact that typically, Martell decks are able to benefit from winning AND losing challenges.