For more powerful riptide cubes (here I use the term to mean "low curve, good fixing, dense synergy" rather than "less powerful than MTGS cubes"), I really like the following cards for singleton breaking:
Imposing Sovereign - white aggro is in my mind at its best when it's disruptive first and wide or tall second. This effect provides a unique form of disruption that complements my next candidate nicely:
Thalia, Guardian of Thraben - she pushes against white prowess decks, but I think most cubes have space for a "heavy creature" theme alongside a "heavy noncreatures" theme in white. She's both an incentive to go into a creature-heavy disruptive build, and a reward late in the draft for already being creature-heavy.
Monastery Mentor - this spot almost goes to Soulfire Grand Master but I want to hold out for some good shard tribrids before I start running multiples of gold-ish cards. Mentor is THE WOMAN WITH THE PLAN when it comes to prowess in Cube decks and having multiples of her in the draft will get your drafters to strongly consider grabbing those enchantments and combat tricks so they can make this amazing card even better.
Cataclysm - much like Balance and Upheaval in GRBS cubes (like my own), this is a single card that brings its own deck to the table just by being in the draft. You can do some subtle things with the rest of your cube to support that deck if needed but if you're a riptide architect you've probably already done them without trying.
Jace, Vryn's Prodigy - has more lines of play (and more power) than any other looter in existence. If you like any of the following: spells, graveyards, modal cards, delve -- you'll like having multiple FlipJace.
Snapcaster Mage - the "business end" of the above's flip side, this bad bruh helps enable graveyard decks, flash decks, prowess decks, even blue beatdown decks. After all, flash is the blue haste! And if that's not enough to sway you, blow up the art for this card and check out those cathedral spires in the background. Drool.
Vendillion Clique -- do I even need to explain this one? Other people have already done so in better words on this very forum. I'll just say the Valley Girl Clique is the end of evolution for "smart aggro" creatures.
Condescend and Remand -- I group these together because they're both summed up the same way: stronk as bonk but in a way that encourages high-level thinking from both players.
Dark Confidant -- perhaps the world's most misunderstood cube card, and I don't claim to fully grasp His Bobness myself, but I will say he opens up a lot of things for a color that tends to be all about a couple fairly simple strategies (reanimation and recursive beats). Bob is the incentive card life gain decks are looking for, he's the reason to splash black in your Courser/Abbot deck, and oh did I mention he draws you a card every turn? Great in aggro obviously, but in the low-curve Riptide control decks that are easier to draft than you might think, he's also a bob-bomb.
Flesh Carver -- who says commander cards are fake? This guy keeps it real, going great in token-sac, ROCK, evasion, pod, and idk, probably some other decks too. And yes, it's because he's a high-value individual card, but it's also because he has so much synergy with things black decks want to do. And that's why you want more cannibals in your cube: fresh and free-range synergy.
Profane Command -- is it a finisher? Is it a value removal spell? It's both, and you can cast it for good beats at several points on the curve. X=1 is fine if you just need to push through one big mamma-jamma and get your Deathrite Shaman or whatever back. You know what else is fine? X=6 to get back Big Tree-Witness and do it all over again next turn. Tasty profane value.
Everyone knows about YP and the good Abbot already so I won't waste text on them. Just go multi on em.
Feldon of the Third Path - ever get tired of turning efficient red dudes sideways? Feel like there's something greater you can be doing with the color of art and romance? Why not run multiples of a romantic artificer who gives you artifact and graveyard synergy, plus a range of finite combo for your drafters to enjoy piecing together! (My favorite is Triskelion, but Precursor Golem and Pia/Kiran are swell too. And lots of other peeps!)
Tormenting Voice - say what you will about WOTC, but they did one thing right this year. They kept T-Voice in standard. This card is a fine little digger on its own, but it's got so many ways to become a value engine. For example, these new "soul" cards in SOI. It's everything Faithless Looting aspires to be.
Birthing Pod and CoCo -- another two I don't feel the need to argue for, since those arguments are public knowledge.
Lotus Cobra -- yeah yeah I'm a power maxer, but you tell me how dirty and delicious it is to have a legit 5-color enabler in your 5-color aggro deck... Twice! The Cobra enables a lot of cube deck brews (most of them involving 4+ colors) that are really hard to enable otherwise.
Nest Invader -- this little card that could is the kind of card no one respects in draft, but once you get to deckbuilding you realize "hot dang, the ramp, the token production, the ETB effect, the free death trigger... This guy synergies with everything at least a little bit!" And so he/she/it does. A great glue card that brings a lot of play in a nice, simple package.
Bonesplitter -- I heard you want to encourage aggro. How about encouraging aggro while also enabling Trinket Mage to become an aggro engine? Oh, and it gives Stoneforge Mystic something to fetch if anyone's still running her.
Phyrexian Revoker -- this one is very format-dependent but if you have the targets, this card is absolutely the zen maindeck that can give your opponent a different headache every game.
Precursor Golem -- Such a weird card! What does it do again? Well, it gives drafters who want a different kind of combo deck something really spicy to try, and if you run splicers too, the fun just gets funner. You don't even have to run Become Immense to make these guys silly. Try any cantripping combat trick for a build-your-own Ancestral, or for control players, target these guys with Mystic Confluence or Profane Command for ULTIMATE VALUE.
