Past in Flames
 
Flashback is one of the most Spike-centric mechanics ever invented. It's nothing but card advantage, casting the same spell twice with one card, and while that may not be interesting to you in and of itself, a card like this is very effective at getting you to wonder about what might happen if various spells that didn't have it suddenly gained it. The cost is a little on the high side for casting a bunch more spells from your graveyard in the same turn, so it's probably best used mainly in decks that are actively designed to take advantage of it; you probably don't gain a whole lot from putting it in a more straightforward style of deck.
 
Constructed: 3/5
Casual: 4/5
Limited: 3/5
Multiplayer: 4/5
EDH/Commander: 4/5

Past in Flames (3/21)

Storm’s best friend, Past in Flames is the definition of a finisher. The main aim with it is to, after flooding your mana pool with cheap mana, reflood it and then go into a Grapeshot or Empty the Warrens. It's not a hard spell to work your way into in the right build, and giving everything flashback is powerful. It can even flash back itself, so you have two attempts to go off if one misfires.

Past in Flames has the downside of being a narrow-ish card. It really only works in decks that can generate a lot of fast mana, because 4 mana to turn on flashback means you're not getting a ton of mileage out of a conventional set-up. It also only affects cards already in your graveyard. Make no mistake, Past in Flames is powerful, but it has a lot of demands in deck-building.

Constructed: 3.5
Casual: 4.5
Limited: 2
Multiplayer: 3
Commander: 4