Toruviel, she has stolen quite a few games for me. I´m focusing NR at the moment so I could also say PAVETTA! Works great against Nilfgaard and I just love it when she screams and cards start to fly. I reckon people don´t really run her in decks 'cause everyone seems to spend a few moments reading the card afterwards and I haven't seen anyone else use her.
The card I play in most of my decks these days is Geralt: Igni. It's such a solid addition to any deck, especially if you don't have a lot of removal. Even when you do this card can instantly win you rounds when your opponent isn't expecting it.
Monster is c harpy, wh hounds, frost, sk is bear, shield maiden, hjalmar. Renew and royal decrease are both great cards. After the patch with bears everywhere, c harpy is weaker so I go for earth elementals more. Mage is great as well. Nearly always play them. Weather clear is king.
Saskia is especially good with Francesca as you can mulligan her at the beginning of the match if she's in your hand, boost some units and then play her along with the leader. Another super use of her is with Sarah. If both are in your hand at the beginning of a match, it's good to keep her and get another gold in her stead. She's never lost. I agree Order cards are good and when they are gold, all the better.
Neutral: First Light which is both versatile and really a necessity thanks to weather. Igni is the best creature, though regular plain geralt has been a great help many times (as long as the opponent doesn't have renew).
Monster: Ancient Elder. He's a wonderful insurance card for when someone invariably spams weather or if you're basically in trouble and need some backup. Also great against spies and cows. Special mention to the REGULAR harpy! She makes Death Wish fun.
Nilfgard: Imperia Enforcer. Spies got you down? Not any more! Fun with Iris, too. Sweers is a close second, but still really nice. Being able to kill all of a super annoying troop (ex: nekkers, reaver hunters, ect) is a wonderful utility. The only time he ever backfires is if your opponent is Skellige.
Soai'tel: Baraclay Elis never dissapoints. Ever. Also Nature's gift to get anything you need right when you need it.
Skellige: Shield maidens are fun for poking berserkers, turning a damaged/weathered troop into more points, or killing spies. They aren't the easiest to use, but when they work they're wonderful. Blueboy Lugos and his whale are a little risky thanks to decoy, but if the opponent already used decoy to rescue some "important" troop they are in trouble!