Pyromancer Ascension
 
When Zendikar was new, this was immediately recognized as one of the easier Ascension cards to activate, and perhaps the most powerful. Of course, that also had something to do with Time Warp being available. It kind of doesn't matter that much what else you're doing on those extra turns when you have three turns for one of your opponent's. You don't have to use an effect quite as extreme as that, though - there are powerful instants and sorceries both in earlier sets and in very recent ones, many of which can win a game when cast even once, much less twice.
 
Constructed: 4/5
Casual: 4/5
Limited: 2/5
Multiplayer: 3/5
EDH/Commander: 3/5