Things are about to change, but as it currently stands:
Ambuscade will be the new 1. Depending on if he gets “combos” off (or not), he can be a chump or moderately dangerous (nothing to worry about).
Baron Blade is getting changes and is more dangerous now. (!!!)
If Apostate is kept as he is, he will be the new 10. He is horribly, horribly strong. A guy I play test with was joking that he’d win vs the Organisation and the Matriarch united against him!
Akash’Bhuta is like an epic boss battle you’d find in a RPG. You have to keep going and going and going, whittling her slowly. And just when you think you have her under control, BOOM! boss changes form and becomes more dangerous, spouting 8 tentacles like some Final Fantasy game! I played 2 games against her today, and it nearly took me 4 hours! (of course, I was taking copious notes, so that’d account for some time) If you have AoE, she’s slightly easier (normal: 6~7ish). If you have no AoE, she’s tough (advanced: 10). (edit: forgot to mention: she depends on the environment slightly more than other villains, so that can change difficulty as well)
Ennead are solid. They’re very flavourful villains and lots of fun to fight. I’d reckon they’re 6~7 normal / 8 advanced.
I’ve not played against Gloomweaver yet.
The Tomb of Anubis environment is very flavourful and ADDS GAMEPLAY TO THE GAME. NEW MECHANICS! Yeah, it will totally change the way you look at environments.
I haven’t ventured into the Planes of Discord yet.
Nightmist is fun to play, never really runs out of options. Nightmist does a little bit of everything. Utility with decks, environments, ongoings; damage, healing; drawing cards, etc. She is a jack of all trades.
Argent Adept is a more nuanced hero and resembles a bard. I think he suffers a bit when he faces villains who destroy equipment and/or ongoings. He has some really great cards (and absolutely rules the environment), but his damage is lacking. Imagine someone like Visionary, except with sustained heals (!) and more complicated cards.
Unity is a botmistress. Spawn bots, more bots and even more bots. She suffers when the villains throw out AoEs or do lots of damage to Lowest HP Hero targets (naturally). But even in the worst of cases, you can see her dealing consistent damage. From the few times I play tested her, I tended to run out of cards to play in my hand (esp. when my bots were being destroyed willy-nilly).