Yencon, beyond any doubt.
The real problem is that not even a Gold card can properly counter her, so it's a no-brainer to use her.
Despite the low scrap cost (200) it's way more powerful than some whose price is 800.
Also, removing her from the field requires Dimeritium+Alzur, something that takes one turn too many given the fact the enemy's deck is often fully prepared to take advantage ot that time frame to either buff her to bleed more cards or outright beat the hell out of your existing strategy.
I have personally found that even if you are prepared to sacrifice 2 cards (often it's a Dim Bomb, which could have much better uses), it is unwise to kill her if she's silver because the enemy will resurrect her asap.
I once played with a particularly cheesy individual, who used Yencon in the 1st round, Renewed her the following round, then resurrected her again after I Alzured her.
(that time I was tempted to ragequit Gwent until the problem was solved, but I cooled down)
Killing her is not just too expensive, it's unwise.
Even Caretakering her after killing lacks punch, because the opponent easily Alzurs her on the spot or uses other pest-removal devices who do the job perfectly considering you do not have to remove the Gold.
My suggestion to balance her would be to have her ability trigger every 2 turns, so that there is no risk of systematically leveling the field. Making her Nilfgaard-only, too, might help.
Cahir, because the enemy has the means to use him with impunity if carefully timed.
He should be allowed to use his ability only provided the enemy has not passed and can use their leader again.
Radovid.
8 points of damage are not critical, the problem is that it's too damn convenient.
Gold cards are impervious to most harm by design, and all of them who have an ongoing effect on the battlefield are within 8 points of Strength.
Dagon is a little more than Aeromancy, which is silver, and has no power at all over Gold.
Radovid guaranteers the demise of the enemy's best Gold unit, the fact he can snuff Yencon is why he's so popular in my opinion.
I would change him to "deal 8 damage to a Bronze or Silver card, or 6 damage to a Gold card". That way, enough Golds would be safe from insta-kill.