Super-Kings or Super King Chess (Monster chess) or Racing Kings: which is the hardest variant?

The Super-Kings chess variant is where the King has all the powers of a queen and knight, and can move and capture exactly like a queen and knight. Outside of that, the same rules of chess still apply. Jose Raul Capablanca was a genius at these types of variants.  This fun variant has been played in different forms for quite some time now! It is being played a lot in chess clubs today!

Super King chess or Monster chess is a chess variant in which the White side has only a king and four pawns to fight against all the pieces of the Black side. All the rules of chess apply, except that White makes two successive moves per turn. The white king can move into check on the first move of the turn and move out of check during the second move. The goal for both sides is to checkmate the opponent's king.

Racing Kings is a chess variant where the goal is to be the first player to get your king to the eighth rank. The game uses a special starting setup on a larger board, and kings can never be put in check, which means players cannot make moves that threaten the opponent's king. White can win by reaching the eighth rank, but if White does and Black reaches the eighth rank on the very next move, it is a draw. 
 

Which variant is the most difficult to play? All these variants are so fun to play!