Who is the best character in Slay the Spire and why?

The only character available at the start, the Ironclad is what could be considered a stereotypical warrior class. Not only does he have the most starting health, but his Relic heals some of his HP after each battle. A majority of the Ironclad's abilities are based around dealing damage and weakening enemies physically by lowering their attack power and defense. He also has cards that allow him to raise his max HP and heal through attacking. Because of this, the Ironclad could be viewed as the easiest character to play as his deck allows him to tank hits while attempting to overwhelm enemies with hard-hitting attacks.

The second character you get access to, the Silent is unlocked after you complete a run with the Ironclad (though you don't have to reach the top). She has the lowest starting health of any character but her Relic allows her to draw 2 extra cards when combat starts. This character is based on hunters who use a series of weapons and traps to wear down their foes before they move in for the kill. In terms of the Silent, these tactics take the form of cards that allow you to build up a lot of low-cost attacks while gaining the ability to poison targets which deal damage overtime and bypasses Block. Because of this, the Silent relies a lot on building up their arsenal and cards that have delayed effects. This character needs to be played conservatively but if done right, can build up a lot of damage over time.

The third character you can play as, the Defect is unlocked by finishing a run (not achieving victory) with the Silent. It has the second highest health score and a Relic that grants a Lightning Orb at the start. This orb-based robot uses cards that make it seem like it can use magic when, in fact, it is manipulating the energy within the orbs it gathers. The Defect has three orb slots over its body which can be filled by playing orb cards. Some orbs require cards to activate them while others activate as soon as the turn ends. They're all based on various elements which grants them a range of effects from dealing damage to providing energy. As a result, the Defect requires time to set up a combination of orbs that will provide the most passive effects while you concentrate on using other cards. It also means you'll need to decide when it makes more sense to strengthen the orbs for later or use the cards you have on hand.

The fourth and final character in the game, the Watcher is unlocked by completing a victory run after you've unlocked the Defect. This character has the second lowest starting HP and a Relic that adds a Miracle (retain one energy point) at the start of combat. Inspired by martial artists, this character incorporates a lot of Eastern practices into their fighting style. This noticeably takes the forms of Stances, which the Watcher can enter or exit by playing certain cards. Each Stance provides different bonuses while it's active, such as Wrath doubling damage and Calm providing extra energy upon Exit. The crown jewel is the Divinity Stance which grants you three energy points and lets you deal triple damage. However, this requires you to have cards that allow you to build up Mantra which is needed to enter this Stance. This means that you'll need to go on the defensive while you save your energy to reach more powerful Stances so you can dish damage capable of killing many enemies in one hit. This, effectively means that The Watcher is the best character in Slay the Spire, without any doubt.