Which card do you always want in a rainbow deck?

[Silver]

A cheap, cost efficient attacker which is even more valuable in this denial filled meta allowing you to play it off scraps. RT resistant affording you defense against the common Ghostmare and every other deck that uses Reverse Time or Eternity. Its low damage commitment means you'll have an easier time dealing with Antimatter and SoSac.

[furballdn]

A cheap, cost efficient attacker which is even more valuable in this denial filled meta allowing you to play it off scraps. RT resistant affording you defense against the common Ghostmare and every other deck that uses Reverse Time or Eternity. Its low damage commitment means you'll have an easier time dealing with Antimatter and SoSac.

While very true, most people would much rather an arsenic, skull shield, bonewall, or flesh recluse over a skeleton. Arsenic costs only 1 more , yet does double the damage, and adds poison as well. Your point on RT resistance is very good, but flesh recluse isn't expensive either, while dishing out three times the damage of skeleton. Main problem with its low damage output means that it's really slow (in a game about fast speedbows), and susceptible to shields.
If you got offended by the above picture, I do apologize. I did not mean to offend you, and perhaps in hindsight, it was a bit harsh of me to post that. Take it as a joke if you will. I'm also the one who gave the troll answer of nova and QP

[Acsabi44]

everybody who names cards, is mistaken.
there are at least two disctinct types of rainbows:
speedbow and
stallbow.

The two decks are fundamentally different.
There is not a single card that is a necessary component of both.

The closest thing would be SN, but there are several stallbows out there without SN.

[Mathematistic]

A cheap, cost efficient attacker which is even more valuable in this denial filled meta allowing you to play it off scraps. RT resistant affording you defense against the common Ghostmare and every other deck that uses Reverse Time or Eternity. Its low damage commitment means you'll have an easier time dealing with Antimatter and SoSac.

Mummy has RT resistance (at least by AI) as well as being almost as cost-efficient as flesh recluse.

[Silver]

I'm not serious people... Was I that convincing? 

I agree with Ascabi et al in that you can't really name a specific card even disregarding specific combo-based rainbows. The closest thing would be a cheap utility card like Mind Flayer or something. Fog Shield might be another one. Both are cheap and provide significant gains for both fast rushes and slow control bows.

Also how is QT a troll answer? Sounds valid to me. Even with something SNova based QTs add stability to pretty much any rainbow you can imagine.

[Mathematistic]

OK, to hell with trolling.

Best ones in unupped decks: Nova, Fog Shield, Mind Flayer, Graboid, Arsenic
Best ones in upped decks: Supernova, Discord, Pulverizer, Electrum Hourglass, Wings

[Absol]

Already said above. I will probably add Entropy pend to list, but that's just because PSNbow has not been mentioned yet.

[Acsabi44]

QT: PSNbow doesn't need it
Mind Flayer: would be a pretty bad draw in a speedbow
Fog: A stallbow has tons of better alternatives (dim, wings+spider etc.)
Arsenic: No role in a stallbow
Pulverizer: no role in a speedbow

etc etc. not a single card.

[TheDreamingMind]

Let me put my question in another way: What card would you always want to have in a rainbow deck?

[Acsabi44]

Let me put my question in another way: What card would you always want to have in a rainbow deck?

makes no sense either. You can have a stalbow or a speedbow, each requires different cards. You won't be happy if you draw a Fallen elf in your speedbow; whereas it is a nice draw in the stallbow (if you play that variation.)

If you wanna know what cards are useful for your deck, then specify if you want to build a stall or a rush.

[daccoo]

Steal and houerglass

[Poker Alho]

it would probably be Quantum Tower, excluding PSN bows. This just shows that there are no cards that can always fit perfectly in every raimbow and that feels right to me