Modified Atarka Red (tokens)
Rally is the deck to beat, and we all know it. Atarka Red can really only get there with a nut draw. But with a splash of white? Suddenly it's an entirely different matchup. A turn 2 Seeker of the Way into a turn 3 Titan's Strength + Dromoka's Command gives you a huge amount of reach. Gaining 16 life while removing a threat AND hitting them for 8 makes it far harder for them to ping you to death with Cutthroat.
Dragon Fodder & Hordeling Outburst gives you chump blockers and make Reflector Mage far less effective. Currently running 4/2, but might move to a 3/3 split.
Sideboard guide:
Rally - None of their creatures are really annoying except Ayli, but they run far too many of them to ever expect a clear path. Enter Barrage of Boulders.
Abzan - Super easy. Bring in the Roasts and Silkwraps, go wide and go fast.
BR Dragons - Individually, nothing they play (even Kolaghan) is a problem. Keep applying pressure, force them to block. Arc lightning is good against the Thopter tokens.
Atarka Red - You are not the beatdown. Hold your removal and do your best to keep things from getting through. Your Seekers give you a major edge.
RG Eldrazi - Floor it. Watch for Kozilek's Return, but if they're spending their mana burning your board then at least they're not ramping. You will get them low before they can stabilize, at which point Wild Slash, Atarka's Command & Exquisite Firecraft should close out the game.