The way challenging works is a ready (untapped) character exerts and chooses an opposing exerted character to challenge, and then both cards compare their strength (attack) and willpower (defense) values to each other (meaning the attacker automatically gets hit back by the defender)
Honorable Mentions Not all cost increasers hurt your opponentssome tax you instead
Miirym, Sentinel Wyrm Miirym, Sentinel Wyrm is a very strong card and would likely top this list if it werent built for a specific type of deck
Anafenza, Kin-Tree Spirit bolsters onto your persisting creature as long as it has the least toughness when it arrives, which is easy given the smaller size of the better ones
Remember how Kiki-Jiki, Mirror Breaker can make infinite creatures
Good qualities aside, I think this card is too expensive and clunky to truly compete in EDH matches