Not crazy awesome, but crazy funny. Down with the Bunny Empire!

Summon.

A lot of these answers focus on "powerful" however this is explicitly not what the question asked:

16 hours ago, Ungod said:

Not crazy awesome, but crazy funny. Down with the Bunny Empire!

Therefore It is clear that it is the dance spell will make everyone dance, it’s undoubtedly the best spell in MagicDuel

I would say that the craziest spells in MD are the inner magic ones. Although these are not implemented yet as it is still in the game border, when you read what actually goes into creating one, it could transform the game itself. 

There is a lot of work involved in casting these spells. According to the documentation, you literally can do anything you wish for. It consumes a Wish Point and Principle values to cast. The requirements are inner magic documents (the first few levels), the description of the spell, and the how the spell would consume the provided Principle values. 

Taken directly from the inner magic documentation page:

Spoiler   Example Spell requirements Example Spell casting cost Example Spell effects - pages 1,2,3 of invisibility spell - 100 darkness, 150 imagination, 50 transpozition (one principle type for each level) - one wishpoint - " darkness makes me unknown, imagination takes me to the unreal realm and transposition makes it all stable for a limited time. Once invisible i will go in x location and listen to the discussion they have there" - consumes the indicated principle points - takes one wishpoint if successful - player will receive the chat log from the described location or a summary of it, depending on what the rpc decides. if failed - player gets discovered by the other players and they will know he attempted to get invisible/spy. Loses one wishpoint.

If this is not crazy enough, then I don't know what is. It's one of my long time wish to have this feature realized.