Everyone has been talking about Demonic Pact. It’s a strong card as long as you don’t ever have to choose the 4th option. While it has plenty of casual appeal, it’s already shown some potential in Standard at Pro Tour Magic Origins. It’s already seen an appreciable bump in price, however, but if it becomes a strong competitive card, as a mythic you want to run multiple copies of, it could hit $10. Foils are probably a better investment at this point.
Hangarback Walker is pushing $15 by continuing to show strong competitive results. Buylists haven’t quite caught up to the spike, but the demand should keep its price well over $10 for at least a bit. Such a wide range of decks are playing full playsets.
Day’s Undoing and Archangel of Tithes are still popular mythics, but aren’t quite putting up the results thus far to see them as solid investments just yet. Both have plenty of long-term potential, though, especially after rotation. Major buy-lists are still shunning the flip-walkers as their market continues to settle. It’s not a stretch to say though that Jace, Vryn’s Prodigy and Nissa, Vastwood Seer are the top two in Constructed right now. Amazon sales would seem to suggest there’s plenty of faith in Chandra, Fire of Kaladesh, too. She just hasn’t found her home yet. Post-rotation sleeper, anyone?
The best rares in the set as far as spread (retail - best buylist price) continue to include Dwynen, Gilt-Leaf Daen and Scab-Clan Berserker. Dwynen is an intro pack rare with an alternate art foil version, however, which hurts her set foils. Elves may not be a quite a Tier 1 strategy in Standard right now, which could change with new Zendikar cards coming into play in October. It’s still not a bad idea to hold a playset of her as she’ll always have at the very least casual demand.
Scab-Clan Berserker, however, is a creature that really should see play. While he has to have Renown turned on for his effect, he is a 2/2 with haste. If you get him through, he punishes your opponents for playing non-creature spells. While he’s no Eidolon of the Great Revel, he’s strong. There just doesn’t seem to be room in Standard Red decks for him right now, especially with Abbot of Keral Keep deservedly getting a chance. If Red starts playing 4 copies, seeing this as a $5 card is quite realistic. Foils are still fairly cheap at around $4, too.
Honorable Mention: Harbinger of the Tides
The Harbinger looks like it will be a pretty decent card in Standard, but for Modern Merfolk that can pop it into play with Aether Vial, it could end up becoming a staple. Foils are about $15 right now, but that number could shoot up quickly if it sees sufficient eternal play.
Herald of the Pantheon - While it’s seeing some decent amount of play in Constellation decks, that’s not an archetype that’s probably going to hold up in Standard unless Battle for Zendikar and future sets have enough cheap Enchantments. Starfield of Nyx is awesome but it loses a lot of support with Theros block rotating, and the deck may not hold up in Modern (although it may.) Still $5-6 for a foil Herald seems a bit light and they already buylist for $3-plus.
Displacement Wave - Here’s a card that could be very strong depending on the metagame. At the very least I see it seeing some Standard and Modern sideboard play. Foils of these types of cards tend to be far more expensive than you would think. At $2, this seems like a good one to hold onto.
A couple foils that may be running under the radar: Citadel Castellan, Calculated Dismissal, and Necromantic Summons. The Castellan is one of the best Renown creatures, being a vigilant 2 / 3 for 1GW that can become a 4 / 5 with Renown. If it ends up seeing decent Standard play, foils could be a few dollars. Calculated Dismissal is a permission spell that may be too slow for Standard. It’s basically Mana Leak but for 2U that has a Spell Mastery ability (if you have two instants or sorceries in your graveyard) to Scry 2. It’s really good for Commander, though, so foils being only $1.50 seems a bit low.
Necromantic Summons is probably the best uncommon foil investment right now. It’s a reanimation spell for 4B that with Spell Mastery adds 2 +1/+1 counters to whatever you bring back. It’s much too slow for ordinary 60-card Constructed, but plenty of Commander decks can find a use for it. It hits any graveyard, too. At only $1, foils definitely have room to grow. It is a lot like Rescue from the Underworld, which is only a 50 cent foil, but despite the combo potential of Rescue, Summons is more straightforward and should be good in the format. Worst case, it drops to a 50 cent foil, best case it becomes a few dollars.