First off, there isn't just one way to play each class Shadowverse. You can have 40 cards in a deck which means you can customize your deck depending on your play style. That being said, the list of cards that you'll see here might or might not make your deck stronger. Keep in mind though, these cards were assembled to increase your winning percentage depending on your chosen class' strength. So without further adieu, here are the top 2 cards for each leaders in Shadowverse!
Top 2 cards for Forestcraft Deck:
Most cards on Forestcraft will let you draw Fairies. Its main strength is the ability to drop multiple cards in a single turn to overwhelm your opponent with numbers.
Rose Queen: Fanfare: Transform each Fairy in your hand into the spell Thorn Burst.
This card greatly plays inline with Forestcraft's strength. Coupled with a deck that lets you draw multiple fairies, this legendary card is really overpowered especially with Fairy Princess. Thorn Burst is a 2-cost spell card that deals 3 damage to an enemy, you can cast this spell directly to your opponent's leader which means, if you have many Fairies in your hand and plays the Rose Queen, it will pretty much end the game.
Titania's Sanctuary: Can Evolve an allied Fairy each turn for 0 evolution points.
You already have an idea what the Fairy Princess can bring to the table, but it's already mentioned above plus it seems cheating to still add Fairy Princess on this list since we only got 2. This card is really good especially mid game, Fairies will be plenty dangerous with this card and this gives you an advantage late game as well when your opponent doesn't have any evolution orb to play.
Top 2 cards for Swordcraft Deck:
Swordcraft's cards have traits namely, Commanders and Officers. These cards relie on each other to stronger as a whole team. There are many variety of decks that you can build on this class from cheap costing to mid ranged, from swarm to quality over quantity. Some cards can summon multiple units on field while some have special abilities like Storm. Either way these 2 cards can fit on your deck no matter what play style you chose in this Shadowverse class.
Sea Queen Otohime: Fanfare: Summon Otohime's Bodyguards until your area is full. (An area can hold 5 cards.)
This is a great card to play especially when you're in a pinch or if you want to gain quick advantage early in the game. Couple this with the Admiral and they can all be very lethal. A good way to perform this, is to attack one of the Bodyguards on your opponent's follower to have it die then drop your Admiral.
It's a tie ! I would have put the Admiral in here as well but, again, it feels like cheating, anyway, Sage Commander and Admiral has more or less the same presence. Admiral gives more buffs while Sage Commander has a better individual damage. Depending on the situation, you might need that extra damage from Sage Commander
Sage Commander: Fanfare: Give +1/+1 to all other allied followers. That Fanfare effect is just too good coupled with your other followers, you have the opportunity to launch a big attack. Unlike the Admiral, you can pass the buff even if the followers you have in play isn't an Officer.
Top 2 cards for Runecraft Deck:
Runecraft specializes in magic. Most cards on this class has the Spellboost effect. The more you use a spell, the cheaper and powerful a Spellboost card will be. There are also many variety under this class, some decks focuses on Spellboost while others takes advantage of the Earth Rite effect. These two cards can be useful whatever deck build you chose.
Merlin: Fanfare: Put a random spell with Spellboost from your deck into your hand.
Whenever this follower attacks, spellboost the cards in your hand. This card's effect is really great. A free Spellboost card and the potential to boost the cards in your hand, can give you an edge whatever type of Runecraft deck you are using.