I am going to play 4- color Rally the Ancestors combo at Pro Tour Battle for Zendikar. The list isn't set in stone yet, but here's where I'm at right now (the day before the PT):
4-Color Rally The Ancestors
This deck builds up a presence of creatures on the board and in its graveyard using Jace and Collected Company and then finishes the opponent off with Rally the Ancestors. It doesn't one-shot kill like previous Rally decks could, but instead uses Zulaport Cutthroat to drain the opponent to death. Between draws from Grim Haruspex and scrys from Catacomb Sifter, it is easy to chain one Rally into another. The deck does occasionally win by attacking with a large Nantuko Husk, but they often have enough blockers to make this tricky. Sidisi's Faithful can help you force the Husk through.
Speaking of Sidisi's Faithful, this is a card that probably not a lot of people expect to see play at the Pro Tour. The suggestion came from Michael Jacob, who was helping us test in Denver, and it was a great one. Sidisi's Faithful is a cheap defensive creature that can be used to create big tempo swings. It also cycles when you have Haruspex in play, and can even be used to re-buy Visionarys in a pinch. Another cool use of Sidisi's Faithful is with Company. Surprising someone in combat with a bounce effect can be sick.
Playing the deck is a bit tricky, but it isn't as hard as some other combo decks. If you've ever wanted to play combo but have been reluctant because of difficulty, this may be the deck for you.
You have a few main goals when playing the deck: the first is to stay alive. The deck is not particularly fast, so making sure you can buy enough time to get everything set up is crucial. The second is to find the creatures you need. Nantuko Husk is the most important creature for combo’ing, and Zulaport Cutthroat is the second most important. Haruspex is nice for chaining Rallys, but Catacomb Sifter can also function in that role. Last but not least is to find Rally. The way the deck is built, it is very hard to win a game in which you don't cast Rally. Once you have the creatures you need for the kill, just keep scrying and cycling until you find a Rally, cast it, and win the game.
• Rally is a bit of a nombo with Jace. Rally exiles itself when it resolves, so you can't flash it back with Jace. The good news is that if Rally gets countered or taken with a Duress it does still go to your graveyard. If it resolves, you can probably win without flashing it back.
• Keep in mind that Catacomb Sifter can make mana if you return it with Rally. Casting Rally for 3 on 6 lands and then using a Scion from Catacomb Sifter to cast an extra Zulaport is a sweet line.
Rally is certainly an exciting deck, but it does have some pretty big vulnerabilities. For starters, Anafenza is nearly unbeatable. Almost every card in the deck cares about going to the graveyard, and only Sidisi's Faithful answers it in the main. It is pretty easy for an Abzan deck to leave up removal and counter the Faithful's exploit, leaving you defenseless. My hope and belief is that most people on green/white aggressive decks will not splash Anafenza.
Counterspells are also tricky to fight through. Collected Company and Rally are both crucial to the deck, and both get countered by Dispel. When decks that don't apply pressure play these cards you can often build up a big enough hand to fight through them, but this gets a lot trickier when you are also getting beat down by a Mantis Rider.
Pre-board, the Jeskai matchup is pretty good. They can pressure you with Mantis Rider, but they don't have enough countermagic to back it up. Once they bring in counters, it gets a lot harder. You can bring in Dispels to fight back, but you also need answers to Mantis Rider to relieve that pressure.
It seems like you might be able to gum up the board against red, but a lot of them are running Temur Battle Rage. Because of this, the matchup is pretty rough pre-board. After board, you have Offshoots to buy more time and Murderous Cut to interact with their pump kill.
Esper has quite a few counterspells, but they don't apply much pressure or defend very well against your random creatures. Use the pressure from Catacomb Sifters and other random creatures to force them to tap out, and then punish them with Rally or Collected Company. After board they have more cheap counters, but you get Fleshbag Marauder to fight against the little pressure they do have.
Change nothing if they don't have Anafenza.
If they do:
How this matchup goes depends heavily on whether or not they have Anafenza. If they don't have it, it is very easy to gum up the board and buy time to Rally kill. If they do, you have to try to set up a turn where you bounce or kill it and go off, but it's a stretch.
As you may have noticed, I tend not to bring in many cards. When playing combo, less is more. Diluting your deck is a major concern. In particular, be very careful about how many Company hits are in your deck. There are currently 27 hits in the main deck, and I think you can go down to about 23 while still feeling OK about Company. Any less, and you have to start boarding it out, which is a major cost.