here's my list of most dangerous, game changing cards:

-holy awe (that extra turn is all your opponent will need)
-burst shot (holy awe and destruction rolled into one!)
-searing wave (king of field control)
-crystal lancer (cant hold this back-2 shield break is always gonna happen and is all you need to win the game)
-mist rias (shouldnt count, but ive turned many games around with this.)

those are my top 5 game changing cards, in order of how devestating and sudden they are.

Bryan645:yeah it probably is if YOU have it

I was playing my friend (he had two shields left), had the perfect setup to win on the next turn (a monster with double breaker and one without), and he plays SUNDROP ARMOR!

I HATE THAT CARD!!!!!!!!

If it had to be one card that changed the game the most in any game I've played it had to be "Holy Awe" as others have stated. The last thing you want is a scenario like this:

You're feeling confident with a nice wall of blockers and a field flooded with weenies and maybe a few powerhouses even if you're sitting there with no shields. Your opponent has maybe one or two creatures that you know wouldn't be able to get through your defenses. You decide to take the win by going for that last shield and as your opponent picks up the card he begins to crack a smile. You see a quick gleam in his eyes as he flips the card over to let you see and wouldn't you know it: HOLY AWE!!!That one turn is all your opponent needs to get the job done.

It hasn't happened to me to often but that scenario worked in my favor as I won with just a "Deadly Fighter Braid Claw" that day. Hah, the look on your opponent's face is rather priceless.

DemonCommand: Hey i like greenday too but the most annoying card or cards i think are holy awe the most ovious chose stated as well as terror pit a $hit of a card!

Holy Awe...

The Gloriful Card from the Gods and/or from WoTC...

I won with that card and I lost to that card...