Today, Standard is a diverse environment with plenty of choices to be made. Fast aggressive options, powerful midrange decks, and slow control builds are all viable choices with demonstrated success.
The equation changes slightly with Team Unified Standard.
Each team must supply three Standard decks. The caveat, however, is that the four-of rule for the format is applies across the decks, not just within. This typically means two players can't bring the same deck since most want four copies of key cards. Finding archetypes compatible with each other adds another level of challenge to the mix: you want decks that feel good, seem great against the metagame you expect, and they have to all be sufficiently different from each other to function normally.
Fortunately for players, between Khans of Tarkir, Magic 2015, and Theros Block there are so many nonbasic lands the problem of mana was more or less lifted. The result was that countries could play multiple decks of many colors, leading to this total breakdown:
For every archetype note that that the count of decks is also the count of unique countries playing the deck: There are no duplicates within teams. However, looking at the top nine archetypes by count covers 80% of the field:
Far and away, Mardu Midrange was the most popular choice for teams. At just about 53% of countries, you can expect to find Mardu in half the matches happening in any given round. Powering out Stormbreath Dragons, Butcher of the Horde, plenty of tokens and some burn to spread around, Mardu has a strong first game and surprising reach even in later turns with Planeswalkers.
Close behind is an archetype that, on paper, shares a lot of colors. The fact is that the cores of the decks are so far apart it's easy to build both within Unified Standard, and many teams did. Abzan Midrange, made famous by Ari Lax and others in the Top 8 of Pro Tour Khans of Tarkir and chosen by Day One undefeated World Championship Top 4 contender Patrick Chapin, is well-established on the week. Siege Rhino, Wingmate Roc, and powerful removal give it the ability to not only stabilize against more aggressive decks, but also reach in later turns with Planeswalkers.
Throwing the Whip of Erebos-inclusive version into the mix makes Abzan (the colors) even more popular than Mardu, but the versions running the potent artifact enchantment borrow more notes from Black-Green Constellation and Sidisi Whip decks than the tried-and-true Midrange flavor. Both of those decks run powerful creatures and self-discard effects to take advantage of the ability for Whip of Erebos to grind out a long game. Twisting towards card draw and disruption or a heavier self-mill theme with an extra color is the biggest distinction between the two, and both were popular choices for the Standard portion of the World Championship Swiss rounds.
Two decks near the top in popularity are also two that are very different from the Mardu and Abzan options above. White-Blue Heroic is a breakout from large Standard tournaments, built to use heroic creatures like Hero of Iroas and efficient bodies like Brimaz, King of Oreskos next to previously-most-seen-in-Limited spells like Stratus Walk and Ordeal of Thassa. Speedy and surprisingly resilient, auras chain together to draw cards and create monstrous-in-size creatures.
Temur Midrange is the other deck near the top, and it's an animal of a different sort. Creatures like Heir of the Wild, Savage Knuckleblade, and Polukranos, Word-Eater are all extremely efficient for their mana costs. Paired with burn and cheap creatures that produce mana, the inertia of attacking with big bodies can overwhelm other, slower decks as well as apply burn to end a game on the spot.
The other two decks left unmentioned Are Jeskai Tokens and Blue-Black Control. The former was the Standard weapon of choice for World Championship Top 4 contender Yuuya Watanabe, a "go wide" approach that made non-combo use of Jeskai Ascendancy to power up smaller creatures to lethal proportions. Blue-Black Control was played by Pro Tour Magic 2015 champion Ivan Floch in the World Championship, and it's a deck with access to tools not in other decks.
And that's the secret here: outside of the most popular decks of all, many are simply the best answer available from the remaining pool of cards. Thanks to the World Championship rounds earlier in the week, a selection of diverse decks and cards were on display and discussed thoroughly. The teams that arrived in Nice were prepared by the world's best putting in work ahead of them.
How those that followed the decks of the World Championship fared is a story for another day.