Standard has always been a rather volatile format, and this environment is no different. Since Odyssey's debut at the Invitational in early October, the Standard metagamed has gone through a huge number of changes. South Africa saw R/G beatdown reign supreme, despite Jon Finkel's smiling face being in nearly every deck, leaving many to wonder if the sun had not yet set on the days of Fires. Next in line came the State Championship tournaments, where both Jon and the growing herds of elephants fell before the wrath of Flametongue Kavus, so powerful in this creature-heavy format that many decks splashed red simply for their inclusion. States saw Flametongue decks and decks that punished their opponents' Flametongues by offering up no appealing targets rule the day, with G/U/R red decks in various iterations posting the most impressive numbers. Now the format has morphed once again, and coming into the weekend G/U/R was likely considered the deck to beat. It's remarkable how quickly metagames can shift, however, as the Gateway has produced an U/B Psychatog deck that is easily the talk of the tournament. In the wake of this chaos, it will be interesting to see what decks see play in the Masters itself.
G/U/R: These decks tend to fit into one of three molds: Snake-Tongue, Opposition, or Threshold. All take advantage of the powerful synergy between Fact or Fiction and Call of the Herd, as well as the raw power of Flametongue Kavu. Snake-Tongue uses the Kavu along with Mystic Snake for tremendous card advantage, often in concert with Aether Burst to reuse them as well as to put a hurting on opposing token creatures. Opposition decks have much the same basic structure, but opt for the exclusion of Aether Bursts in favor of the board control of Opposition and generally a more generous selection of flashback creatures, such as Beast Attack. Threshold decks abuse Nimble Mongoose and Werebear for undercosted aggression, often including Tolarian Winds and Wild Mongrel to hit threshold quickly.
G/U/B: These designs choose the board clearing of Pernicious Deed and the enormous size of Spiritmonger over Flametongue Kavu, in large part to combat the G/U/R decks. Very powerful in the matchups it's designed for, but with much less efficient spells than other control decks, leaving it to play catch-up.
G/U/W: My personal project since I saw Mystic Enforcer, this deck combines the most undercosted creature in the environment with efficient counters and removal. Particularly powerful against mid-range utility decks, it can win seemingly unwinnable games on the simple power of the "Dragon".
U/B: The most recent addition to this deck, Psychatog, provides an enormous body against decks that rely on damage-based removal, as well as a huge threat against other control decks with its ability to kill in a single turn. Two of the top four decks in the gateway were Finkel/Psychatog decks, and given the predilection of many pros toward control decks, it's very likely that comparable numbers may be seen in the Masters series itself.
U/B/W: Finkula or Desolation Angel based, these decks have waned in popularity due to their difficulty to keep up in an environment steadily increasing in tempo. Despite this, many players will have difficulty saying no to playing with the invitational cards from the last two years.
G/R: Simple aggression will simply not go away. A number of players ran extremely fast R/G beatdown decks in the gateway, some going so far as to include Overrun, and one - Steve OMS - was rewarded with a Masters berth. Few decks can put a hurting on a mana-stalled opponent as effectively, and few decks can punish a slow start as painfully - beatdown will always be a top deck.
Balancing Act: Spawning a massive online craze after performing amazingly well in a number of online and JSS tournaments, this deck is as powerful as it is confusing. While extremely powerful against decks with no way to stop its "combo" of Balancing Act/Terravore, it's less potent in a field packed with counters and Duresses.
Battle of Wits: At least one player in the Masters series will be sleeving up 200+ cards, and it's not just on a dare. Battle of Wits decks include nearly every good card in the format, and while they can clearly be a tad on the inconsistent side, their ability to produce an answer for any situation as well as their unpredictability give them a shot at taking the prize.
This is merely a summary of the most popular and the most unusual decks that may rear their ugly heads in the Standard format, and is by no means an exhaustive list. It would be scarcely surprising to see something entirely unexpected take the prize, given the huge shifts that are characteristic of the format. This Masters series should certainly be an interesting tournament to watch.