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!