A lot of good Brawl cards from Kaldheim have Foretell, which new mechanism fundamentally changes key aspects of the game. Read more here (… real soon now)
Poison the Cup {1}{B}{B}
Scryfall
Instant
Destroy target creature. If this spell was foretold, scry 2.
Foretell {1}{B}
Cheap, fast removal is a universally desirable effect, and the going rate for unconditionally removing creatures is 3. Scry is likewise desirable, but particularly good for Black which often lacks unqualified card draw, and the additional cost of {1} is fair.
You’ll mostly cast it from exile in your opponent’s turn, which is more advantageous because there’s more open information to guide your card choice when Scry-ing.
And since black lacks fast mana, spreading the cost of expensive cards and obfuscating your intentions in the meta-game [qv. foretold article] can be more advantageous still.
This is either good removal at 3, or very good removal with scry at 4. You’ll often play the removal sub-optimally just for the scry, and it’s hard to think of a black deck that wouldn’t want to run it.
Eradicator Valkyrie {2}{B}{B}
Scryfall
Creature — Angel Berserker
Flying, lifelink, hexproof from planeswalkers
Boast — {1}{B}, Sacrifice a creature: Each opponent sacrifices a creature or planeswalker.
4/3
Leaving aside that rather silly name, ‘Valkyrie has two strong static abilities on a decent 4/3 body, with a very valuable Boast.
Flying is scarce in black, Lifelink is often integral to the deck, and 4 is a very good price for both on the same card. In multiplayer games, flying means that Valkyrie can likely attack with impunity to access the Boast.
Hexproof from Planeswalker’s isn’t terribly useful, but there are a couple of potential adversaries in the current rotation (Kaldheim). Niko Aris and Narset of the Ancient Ways are popular and can be deadly if they gain momentum, so the ability might be useful.
The effect of the Boast ability is to force an opponent to sacrifice a creature, the utility of which is very situational. If they have one big creature it’s terrific, but if they a heap of cheap tokens, it‘s irrelevant.
The Boast is useful partly because it’s optional, so it’s good in any deck, but mainly because the additional cost of the ability is to sacrifice one of your own creatures. Since it’s a cost, it can’t be stopped, and that means that in a purpose-built sacrifice deck, Valkyrie will be a powerhouse.
Dream Devourer {1}{B}
Creature — Demon Cleric
Each nonland card in your hand without foretell has foretell. Its foretell cost is equal to its mana cost reduced by {2}.
Whenever you foretell a card, Dream Devourer gets +2/+0 until end of turn.
Scryfall
If ‘best card’ is qualified as ‘ubiquitously useful’ Dream Devourer is going to end up somewhere near the top of the heap. It’s great.
It’s also cheap; its +2/+0 bonus might have some value, and black/white Clerics is practicable build right now. None of which are why you’ll put this card in your deck.
You want to give all your cards Foretell, granting them all the considerable advantages [link] of that ability, and which are particularly useful to Black. It’s so strong you might well use your Grim Tutor to fetch it.
If you’re in Black, and you’re not playing it, you either couldn’t find a copy, or your deck was unusually (not to say improbably) well-optimised. Everyone else will be playing it.
Rise of the Dread Marn {2}{B}
Instant
Create X 2/2 black Zombie Berserker creature tokens, where X is the number of nontoken creatures that died this turn.
Foretell {B}
Here’s another one of these great Instant Foretell cards, where you can tuck them away whenever you have 2 mana spare, and then play them from exile when it best suits the situation.
In this case, you’re going to hold it back until someone wipes the board, then play it at Instant speed in their End Step for only {B} and get as many 2/2 Zombies as non-token creatures were lost.
In most 4-player games you could expect anywhere from 3-10 creatures to have warranted the wipe, the range of value you’re getting from this is from decent, to frankly astounding.
And if you can roll the player who wiped the board into the bargain, it’ll feel even better.
Varragoth, Bloodsky Sire {2}{B}
Scryfall
Legendary Creature — Demon Rogue
Deathtouch
Boast — {1}{B}: Target player searches their library for a card, then shuffles their library and puts that card on top of it.
Finally, another mention of Varragoth, whose ability to repeatedly search for cards is outstanding, and which got a whole post of its own here.