Retreat Cycle: Retreat to Emeria, Retreat to Coralhelm, Retreat to Hagra, Retreat to Valakut, Retreat to Kazandu
I enjoyed Landfall so much in the original Zendikar that I ended up building three wholly different Landfall decks while it was in Standard. Groundswell wasn't great without Landfall active, but it still packed a punch as a combat trick or with Infect. Landfall isn't an ability that seems to add much to the minimum cost of any given card, so getting more value from the keyword is almost always a profitable endeavor. Sure, Groundswell might not quite be Giant Growth without Landfall, but it's better than the long-time classic buff spell when it is, and you know what people do a lot of in Magic? They play lands. Sometimes you'll only drop three or four, but most games you'll be playing out six or more lands, which, when you look at triggers, can mean a ton of value.
The cards in the Retreat cycle in Battle for Zendikar don't do anything on their own, but they add something new to the mix; choice. Over the years, I've loved a lot of cards that didn't do much in constructed play, because they gave the opponent a choice, which competitive players tend to avoid. In this case, you're getting a choice each time you play a land, to maximize the benefit for the current situation. I like them all to some extent, but Retreat to Emeria is definitely a favorite. It's a tad mana intensive, at 4 CMC, but the triggers can be brutal in a number of situations. If you've flooded the board before you drop it, you can buff the whole team for a devastating attack. If you're going Ally tribal, you can proc every one of your Rally abilities every time you play a land. Given how powerful some of these can be - "creatures you control gain Lifelink until end of turn" - getting it for free over and over again is particularly exciting for me.
Justin's note: Retreat to Coralhelm is the sleeper hit of this cycle and honestly could end up getting broken. It seems pretty unassuming, but the first ability lets you do some crazy things when paired up with the right cards. Whether it's allowing for another infinite combo with Kiki-Jiki, Mirror Breaker and Dryad Arbor, or the real excitement is around Knight of the Reliquary - which has already doubled in price. With enough plains and forests you can make a giant Knight of the Reliquary and generate a ton of mana. Some interesting ways you could go with it is to make a bunch of mana, search up an Eye of Ugin, and then fetch up and cast Emrakul, the Aeons Torn. Not bad for a turn 3 play is you lead with a mana dork.
I built gimmick decks around Precursor Golem back in the day, as I expect every casual player did. It's a fun effect to play around with, and makes things like Groundswell even more ridiculous. Zada takes it a step further, copying any spell you target Zada with for every creature you control. It's not always going to have a big impact, but when it does, it's going to be huge. Gargantuan. Eldrazi-sized, even.
A Boros (Red/White) Ally deck, for example, might have a low-end mana curve, so you can spit several creatures onto the board in the first few turns. If they stick around until you can drop Zada, you've got some impressive potential. Center Soul isn't the worst card on its own, but when you can make your entire team unblockable with Protection from your opponent's creatures' color. Since each of the copies are independent of the original spell, you can choose a different color for each of your creatures, allowing you to cast it after blocks to save your entire team, and hopefully take out a couple blockers in the process. It seems incredibly powerful, but more important to me, it seems absurdly fun to build around.
I play a lot of control decks, which rely heavily on either card draw or 2-for-1 exchanges to be effective. If you don't get your draw spells, or you draw only single-target removal spells, things can get dicey really quickly. Quarantine Field isn't cheap, but it is an X-for-1 exchange, and, more importantly, targets everything but Lands.
There aren't a lot of ways to remove a creature, an Enchantment, and a Planeswalker simultaneously. In fact, this may be the only one, excluding the non-immediate effect of Bearer of the Heavens. Now there is, and I'm really excited to see what I can do with it.
Screwing with my opponent's ability to do what they want to do is a big part of my fascination with deck building. You've got cards like Thalia, Guardian of Thraben that makes spells more expensive, delaying your opponent's plans. There are more powerful effects that we've seen as well, like Iona, Shield of Emeria from the last visit to Zendikar. This time, we've got a very similar creature, at the same 9 CMC, but all colorless, allowing you to include it in any deck of any color(s) that can support the cost.
I haven't analyzed the even vs odd casting cost breakdown of cards that see competitive play, but I'm not concerned with that anyways. i just want to cast a Void Winnower and see what happens. The "can't block" effect for even-costed creatures is just icing on the deliciously trollish cake.
I'm not sure that Orzhov (Black/White) Allies will ever be a thing outside of casual play, but I'm definitely going to be tinkering with it in a casual environment. I have loved Animate Dead effects my whole Magic career, but it's most often reserved for decks that discard giant creatures and then reanimate them into play. As much as i like big creatures, I don't like waiting, so I enjoy running a million little guys out there as quickly as possible. They get outclassed very quickly, though, so having a way to bring them back en masse is really valuable. The fact that they'll each have a Rally proc, which, it is important to note, will proc for every Ally that entered, is even better.
You could potentially get... mathmathmath... 64 triggers off this, if you bring back eight 1-drops. I'm not sure what else there is to say, honestly. Except, perhaps, "JUDGE! Slow play!"