Personally, while Defect has been the easiest for me in the beginning, I'm stuck at ascension 16. Winning with the Defect relies too much for me on orbs. If you cannot get orb spaces or concentration that playthrough is usually wasted, unless you get lucky and use some other more gimmicky builds, like a 0 energy deck...
I'm starting to get better with Ironclad, but so far I've had less trouble with the Silent or the Watcher.
With the Silent there are many cards which are nearly a 100% win getting two of them and upgrading them. For example, use Burst and Catalyst+ in the same turn and you multiply venom x9. While Catalyst do exhaust, Burst doesn't, so by having two of them you can multiply venom x81!! (3x3x3x3). If you get any of the relics which gives you a power each time you use 3 attacks, just a single Blade Dance+ can trigger the relic with just one card of 1 energy. Bullet Time is a free "use everything turn" which doesn't exhaust. Corpse Explosion is really good in any fight with variious enemies (if you end getting Donu and Deca you just need to kill one to kill the other, etc). Footwork usually appears on most runs and it really helps blocking, etc.
But personally, the most broken character is the last one, the Watcher. There are many reasons:
* Just abusing the calm and wrath stances, you can usually breeze through the first chapter everytime and against any Elite/Boss. That means you can pick only the cards you want. With other characters, you usually need to pick something to beat the chapter, but the Watcher can kill almost everything with in the first chapter with the initial deck. You get double damage AND two extra energies on the starting deck...
* There are many cards and even a potion for Divinity. This means there are many ways to get 3 extra enegies on a turn AND deal x3 damage in that turn.
* If that's not enough, there are many cards you can retain and get better for the full combat if you retain. That means you could get them and ignore them for many turns and you have an awesome card for little energy. There is a rare power which reduces energy cost of retained cards by one per turn. I've barely lost in any playthough with that card if I have retain card.
* You can scry with many cards. I inittially didn't like the idea, but you can choose to discard the cards you want. That basically means that if you get scry powers you can kill most of the RNG and get the cards you want each turn...
* And if all of that wasn't enough, she has other amazing cards like Lesson Learned (you can easily upgrade the whole deck with that card), Deva Form (which IMO shouldn't have this character, Defect should be the one with infinite energy), Conjure Blade (power up attack and use it on a high energy turn, and there you have an instant boss killer card), Alpha (in short decks you can easily get x50 damage per turn), Brilliance (which is like Blizzard...)...
Usually, on other characters, if you try to build a deck and you don't get the needed cards, you will get trouble. I love the Blizzard decks with Defect, but sometimes you don't get any frost cards, or you don't get Blizzard, or you don't get enough concentration (or any at all), or you get stuck with the 3 starting space orbs. To be truly invincible, you usually need some concentration cards, some extra spaces with cards or relics, frost cards and Blizzard. If you miss one of those requirements, you can get beaten easily in any fight which scales every turn. With the Watcher, you can get mixed decks easily between stances, divinity, retain, scry or other great cards, as most of them help each other, and win. Even on a bad RNG combat, you can retry the combat and try a different strategy with the same cards and win it. RNG kills many other character decks more easily.
IMO these are the reasons why she is the best character: Most cards can be used greatly with most cards, she has a powerful starting deck, she has plenty of extremely good cards, and is one of the easiest characters to kill RNG.
Anyway, is you struggle with any character, you can get some ideas in the Slay the Spire Wiki, on the characters pages, on strategies:
https://slay-the-spire.fandom.com/wiki/Ironclad I have avoided looking at decks online for hundreds of playthroughs, but once you start getting stuck with a character, it could show you how some of the cards that you avoid may be better than you think :) There are many cards I thought weren't that good and they are awesome...
