UR Prowess (Modern)

Card by Card examination:

Monastery Swiftspear and Stormchaser Mage: What to say? They are the bread-and-butter of ALL Prowess-based decks. Your opening hand should include at least one, if not both of these creatures.

Abbot of Keral Keep: Rarely should this be a turn 2 play, unless you NEED a creature on the field (or hope to hit a land). Ideally, you'll want to cast this on turn 3 and then cast the spell (with your third land) you've exiled.

Jace, Vryn's Prodigy//Jace, Telepath Unbound: Card draw! Then when he flips, you can cast a spell from your GY for his -3 ability.

Bedlam Reveler: A beefy 3/4 creature that will almost always only cost RR to cast it (if the game goes on that long), and more card draw! There are only two copies because that is really all you need. You don't want to be seeing this in your opening hand.

Expedite: Haste is great for Jace, Reveler, or Abbot, but you really just want to get the Prowess trigger from this and the additional card.

Lightning Bolt: Versatile. Cleans up early creatures, or it could deal the last few points of damage to a player, plus a Prowess trigger.

Mutagenic Growth: Obviously, you'll be using life to cast this "free" spell. But again, +2/+2 AND Prowess trigger? Both you and you opponent will be less worried about YOUR life points.

Slip Through Space: Same idea as Expedite, a cheap spell that draws a card and triggers Prowess. This has the added benefit of making something unblockable.

Titan's Strength: +3/+1, Prowess trigger and Scry to find that next spell!

Twisted Image: Replaces itself, Prowess trigger, and makes Stormchaser Mage into a powerhouse!

Vapor Snag: Prowess trigger, removes a creature (in the absence of Slip Through Space when you need to attack) and pings for 1 damage.

Temur Battle Rage: Double Strike. Prowess trigger. And sometimes Trample! Win-win-win!

A turn 3 winning hand looks like this (and one I've actually gotten during a game):
Stormchaser Mage, Mutagenic Growth, Mutagenic Growth, Titan's Strength, Temur Battle Rage, and two lands. Draw into that 3rd land and you deal 24 damage on turn 3.

Or, you can still deal 11 damage on turn 2 if you miss the land drop.

Games will rarely go past turn 4 or 5, especially if you have more than one creature out and they're all hitting their Prowess triggers. There are only 18 lands in the deck (since nothing will ever cost you more than 2 mana) and sometimes that still feels like too much if you're drawing those off your cantrips instead of the spells you need.

Ideally, you'll be on the offensive the entire time, putting a fast clock on your opponent, but there are those few times where Prowess will keep your creatures alive. Don't be afraid to slow down a turn and play defense if it means a comeback to win.

For other matchups, the sideboard has a some counterspells to protect you from removal, some extra creatures - one for more card draw and one for more Prowess fun - and additional removal in case your Bolts weren't quite big enough to kill that bothersome creature!

Since Khans, I've been in love with the Prowess mechanic and this is, by far, my favorite deck to play.