With H=4, so far the Dreamer has been the more challenging of the ST villains. But I haven't played against Iron Legacy yet.
I hate Whipacorns ! :) Especially when them come by two, and are joined by a dark hero :( The exquisite dilemna of having high damage group attacks that could save your life and not using them...
Kismet is great fun. I like the choices she gives you each turn, and her theme of luck mastery is very well implemented. I can't wait for her "bad luck" nemesis to show ! A well deserved win for Alex ! "Ooooops, did I hurt you Fanatic ? Sorry, but this Jinx was too much..."
La Capitan has been fun too - I think the best strategy against her seems to be "kill the Captain". Except she has a crew. That keeps returning and hurting. There are times were I think of her as "the chairman - but fun!"
These three villains are now high on my list of favorite adversaries :thematic, fun to play, hard but not too hard. I like ST ! And Miss Information is a perfect villain to add to the mix.
Omnitron-X has been a surprise - I din't expect him to be so effective. It's a great tank, and he can become an incredible damage dealer with the good cards and a lot of ennemy targets... I have to test him against the Matriarch !
I like Chrono Ranger - the bounties are a great idea, and feel like a very unique mechanism. My first use of him was against Plague Rat, it was a memorable fight, as with bounties, infection bonus, nemesis bonus and a little help from Visonary, he was able to deal from 10 to 25 hp damage a turn - until the rat finally put him down. It was really a "nemesis fight" were I felt how much these two were "related" and hated each other.
I have to test the Block further, but it has been a real fun environment to play in : with a little thinking you can manage to have the cards help you a lot, until you make a mistake and the situation runs wild. Again, very thematic.
For now (I only have a few plays and still have to test he time cataclism and IL), I think it is the best of the three expansions : very thematic decks that are tactically fun to play, difficult but not overwhelming.
And then there is "I will show you what Tanking in pyjamas means" Scholar. My new favorite.