This is brought up fairly often, though in the form of combos. Generally people say Fire+Time has no combos at all, and while this is true, Sundials work very well with Bolts and Fahren, and Deflags work perfectly to supplement Time's lack of PC in a rush. In addition, there's the weak bonus a Precog has in an Immorush since it's a very draw-reliant combo deck. We can safely ignore that now because of SoBe.
The difference between a combo and a synergy:
A combo is generally a set of cards that produce a wanted effect when they're all assembled. An Immo rush without Novas or Fire pillars/pends does nothing but pelt you with Photons (and potential Graboids/Gnome Riders) unless it draws Immo. In other words, it's close to useless without the combo.
Synergy is when two standalone effects enhance each other (or, rarely, produce a different effect), such as draw power and anything. As such, Time has synergy with any element, just like Aether does by multiplying that elements' strengths or adding good defence.
There's nothing that says you can't have both; they're just different terms applied to different things. A combo is Pandemonium+Bone Wall, and the two cards also have synergy. A combo deck is a thing because it relies on combos, but inventing a "synergy deck" is a pointless attempt to create a useless term since you want synergy in all decks. Two parts that work fine on their own but even better together. Most cards of good design have some synergy with some other card(s) (you can't just say a card has "synergy" since the word depends on two objects having a relation), since it adds to the strategy element of the game.
So, what element works the least with other ones? What would be the best way to measure this? Perhaps the best possible mono is the one that has the least to gain from going duo. Well, personally I can skip the testing phase, because I have a rather situational reply based in experience.
Specifically, unupped Gravity has the least synergy with unupped Aether. You can Fractal Mercs or Armagios and add a Chimera, but why would you? You can TU Accelerated Armagios, but why would you? You can pair Oty and Lobo for versatile CC, but Oty is very vulnerable and weak CC, so why would you? There are better, more consistent options available for exactly everything that unupped Gravity has.
This is not because unupped Gravity is the weakest mono (though it is), it's because unupped Aether is so powerful it has nothing to gain from Gravity. I learned this synergy lesson during War, making decks that are at least 50% Aether. Gravity salvage was always the most useless, and always got converted. We tried making decks with it, but they were just plain subpar.
As I said, this is situational. Once you upgrade some Gravy cards or allow upgrades at all, it's a different story. Elite Chargers and Guards are both excellent Fractal options, and Oty becomes a -lot- better too. Similarly, Animate Weapon works well with Titan, PU and Catapults or Overdrives. It's not the most efficient, but at least decent.
