I interpret disruptive many ways... if you're thinking pure destruction its cards that blow up insane amounts of permanents/hand/etc. Secondly if you make your players make too many decisions or god forbid MATH. Three to effect the game which undoes anything, especially something that took a long time to play, like an evacuation. Fourth something that alters someones entire playing strategy, making them make bad decisions, or decisions there deck really has no logical way to go against.
the most disruptive spell is :
#1: balance
#2: pox (the 1/3 gets everyone, especially if there are many many permanents in play)
#3: Sway of the stars/thieves auction (nothing is as irritating as completely starting over or having all your earnings given to every other player)
#4: arcane lab/citadel of pain (makes everyone frantically play spells or very few)
Can wreck swarm decks, multiple spell strategies, make burn players hold burn spells or allow attackers to go unkilled.
illicit auction only happens once, its really not that bad. ive seen people pay a max of 4 life.
Ensnare/evacuation/reins of power are all honorable mentions...
allowing to cast a spell for free that allows any attack to stop, or allow an attacker to attacked unblocked with ALL of their creatures (at eot) is pretty damn impressive.