Its very hard to pick one, but one that always does really well for me is actually Sorin, Lord of Innistrad. 4 cmc makes him playable in modern, he immediately stabilizes the board if you're behind with a life link chump blocker, or pumps your board right away if you're ahead. Weaving the first two abilities makes him a powerful long term force, and games that involve 2+ emblems usually end in Sorin's favour. I've never played against him, but as far as I can tell he's a must-be-answered card for my opponents.
Plus he's black white, so likely you've already checked their hand for counters with thoughtseize or tidehollow sculler, or just broken them with liliana otv.
Death Cloud is just an amazing card and has thus far been at the center of my modern brewing ventures. I love strategies that destroy both players but leave you with a slight advantage after.
Snapcaster Mage hands down. It's a flash creature and an extra copy of whatever is in your GY....in ONE CARD! One of the definitive value cards across all formats but it's also just down right fun to play with. 2nd place might be Tezzeret, Agent of Bolas, my favorite planeswalker ever. Awesome card advantage engine, turns dead cards into live ones, and also happens to be a win condition. Honorable Mention: Goblin Test Pilot for obvious reasons.
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Current standard decks:
Gruul Aggro
EDH General:
Sydri, Galvanic Genius
Modern
Jeskai Control
Jeskai Midrange
Affinity
Esper Teachings
Tribal Zoo
Lightning Bolt. Simple, elegant, versatile (if there aren't any creatures on the board, you can always throw it at their face), and defines the playstyle of an entire color. It's not the best card by a long shot (that would be Snapcaster Mage), but it's my favorite by a mile.
RIP Karn EDH
Plus he's black white, so likely you've already checked their hand for counters with thoughtseize or tidehollow sculler, or just broken them with liliana otv.
Resto is a really close second
Death and Taxes
Pauper
UB Teachings
Tortured Existence
Murasa Tron
Modern
Pod (RIP)
Bloom(RIP)
Merfolk
Vault Skirge's FNM promo art was done by one of my professors, so thats cool, but Mox Opal is also really, really pretty.
Gruul Aggro
EDH General:
Sydri, Galvanic Genius
Modern
Jeskai Control
Jeskai Midrange
Affinity
Esper Teachings
Tribal Zoo
For Fatties, my god, Elesh Norn, Grand Cenobite.