Ugin, the Spirit Dragon
If you just want the card that has the biggest effect then Ugin, the Spirit Dragon is pretty plainly your choice. The Pick 1 Dack Fayden will often be used to steal mana sources which could potentially ramp out your Ugin.
Despite that, you’re still going to want to have a good amount of acceleration in your deck to reliably cast Ugin. Eight is on the prohibitively expensive side of things if you don’t end up with cards like Grim Monolith, so you’ll want to value mana acceleration very highly after taking Ugin. Combining that with the Pick 1 Dack Fayden means you’ll be fighting over great blue cards and great artifacts, which are two pretty highly contested categories of cards at most draft tables. Great decks containing Ugin and Dack are possible, it’s just important to know that’s a tough seat to carve out.
Bloodstained Mire
Taking a fetchland early is fairly common in Cube, though I think it’s important when doing so to make that decision with a deck in mind over the notion of taking the land to “stay open”.
Dack decks will generally be looking for Volcanic Island/Steam Vents so Bloodstained Mire can just work in a two-color deck, though picking Mire this highly should be done to deliberately intend to draft a Grixis deck. Dack Fayden is strong enough to show up in any deck that can cast him, and Grixis decks like Reanimator or Storm will welcome the discard outlet and card filtering.
Fiery Confluence
Fiery Confluence is one of those cards that probably looks weird and inefficient if you’re not used to formats as artifact-heavy as Vintage Cube, but it’s great in this environment. I’m not sure if it’s used more commonly to deal six to the opponent in red beatdown decks or to destroy multiple artifacts, but you get a lot out of one card in both of those roles and the card also comes with some flexibility. Many of the best creatures in the Cube also only have one or two toughness. Confluence is just a great card, honestly.
Drafting Fiery Confluence here would push you in the direction of an Izzet control deck, and one that is very good at answering opposing artifacts at that.
Phyrexian Metamorph
Now we come to the actual correct pick if we’re talking about “staying open”. Phyrexian Metamorph either doubles your best permanent or gives you a copy of your opponents. With all the fast mana and powerful permanents in Vintage Cube you almost always get more than you paid for with a Metamorph.
There are very few decks that I would cut a Metamorph from once I drafted it. Realistically, Storm is the one archetype that comes to mind and I generally try to avoid drafting Storm in the current iteration of the MTGO Vintage Cube anyway.
As a quick aside, I didn’t list Jace, Vryn’s Prodigy or Thirst for Knowledge as poll options despite them being solid cards that basically always make decks specifically because any consideration for those cards is invalidated by the presence of Phyrexian Metamorph in the pack. Jace and Thirst are slightly above replacement level card selection, and Metamorph is just a flexible heavy-hitter.
Pestermite
Finally we direct our attention to the Splinter Twin lobby. If you want to draft a Twin deck in a 540-card Cube that means taking the combo pieces early and often. Pestermite gives a lot of direction on how to draft from here and if things go well a way to win out of nowhere.
The downside is that you don’t always see other combo pieces and even when you do you don’t always end up with a deck that is great at assembling those pieces. The Pick 1 Dack offers some card selection, which is what you’ll want most of your non-combo cards to look like in a Twin deck, so this is a solid pick.
I think that the tension in this pack largely comes from the fact that none of these cards ever wheel. Choose wisely, because the card that’s coming back around is Blade Splicer.