Today is the day, the day Gatecrash comes out! It introduces the last five guilds of Ravnica: Boros, Dimir, Gruul, Orzhov, and Simic.
Guild-leader: Aurelia, the Warleader.
Aurelia is a great attacker in her own right, but with the ability to let you have a second combat step, she is invaluable. That ability with the Battalion mechanic is absolutely awesome. On your second combat step Boros Elite could be a 5/5 attacker that costs one mana.
Guild-leader: Lazav, Dimir Mastermind
Lazav could be Rubblehulk, Ruination Wurm, or even Borborygmos himself. He can turn into any high mana creature for just four mana. Also, any creature he is has hexproof, so he can't pay the Ultimate Price.
Guild-leader: Borborygmos Enraged
Borborygmos is a huge attacker and a great way to get lands and deal damage to opponents. He can make Rubblehulk huge, then you bloodrush him to give Borborygmos a great bonus and sacrifice most of your lands to remove blockers. Finally, you deal a death blow to an opponent.
Guild-leader: Obzedat, Ghost Council
Obzedat can deal 2 damage to a opponent on your fifth, sixth, seventh, eighth, and ninth turn and every turn until you win. You might be thinking, why would I exile it so I can deal 2 damage every turn when he is a 5/5? Here is the thing: he will have haste. There are some cards that can kill it, such as Selesnya Charm. In fact Wizards changed from a 4/4 to a 5/5 just so this could happen.
Guild-leader: Prime Speaker Zegana
Zegana and Master Biomancer make for a large creature and a lot of cards drawn. She would be at least a 4/4 Merfolk Wizard Mutant and you would have drawn at least four cards.