#1 - Wind-Up Factory (By Debris)

So far, Wind-Ups have received a great amount of support in every ZeXal set so far. They make card advantage really easily, and Rank 5s even easier that than. Right now, the only thing holding these cards back is their lack of speed, they typically need to minus or pray their monster doesn't die during their opponent's Battle Phase in order to get out the Xyz. They also could use a more reliable way to reuse their effects, but that won't take much at all.

Anyways, after they get these few really good cards, the deck has a few Holos you can look to. The current Rank 5s will get outclassed by Volcasaurus, Wind-Up Juggler isn't that great, Wind-Up Zenmaister is crap, but then we arrive at an amazing card that, TBH, is one of the main reasons why they actually have potential. This card is Wind-Up Factory.

Wind-Up Factory
Continuous Spell Card
Once per turn, if the effect of a "Wind-Up" monster is activated: You can add 1 Level 4 or lower "Wind-Up" monster from your deck to your hand.

As you can see, this card is amazing. It reminds everyone a lot of Black Whirlwind and for good reason. The real difference this card will make once broken is that it will be at 3. This card could also possibly be searched by a hypothetical Wind-Up searcher card, and this card can activate during the opponent's turn with Wind-Up Rabbit (this guy is probably the next best investment, any takers for this article?). Yea, remember when Black Whirlwind was at 3? Lotsa fun. Now imagine if Black Whirlwind had been SUPER. It would have easily been a $15+ card, but seeing Dark Strike Fighter at the time leads us to believe that it could have easily been more. All we need are 1-2 more cards to break this deck, and this card will skyrocket past $3 if it does so.

Buy / Trade for all copies of this card at $3 or less. 
This investment is: SAFE, because it really can't drop much lower than it is right now