#1 Vilespine Slayer

Expansion: Journey to Un'Goro
You're telling me that putting Assassinate on a body actually makes it good? Well turns out, that's true! Assassinate in theory is a good card, but spending 5 mana to destroy a minion wasn't powerful enough to include in decks. The 3/4 body of Vilespine Slayer can trade into minions.
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You can also use Shadowstep to return it to hand, Elven Minstrel to draw it and Lab Recruiter to shuffle more copies into your deck. You can't do all that with the Assassinate spell.
Honourable Mentions
Sonya Shadowdancer

Expansion: Kobolds & Catacombs
Sonya Shadowdancer is a great include in many different Rogue decks, including the powerful Odd Rogue. Getting 1/1 Vilespine Slayers, 1/1 Elven Minstrels and others can be extremely powerful because of how cheap it is. Sonya is very vulnerable, but you can take advantage of her effect because her mana cost is cheap.
Kingsbane

Expansion: Kobolds & Catacombs
Kingsbane is one of the best weapons Rogue has. You can shuffle it endlessly into your deck, keep the upgrades from Deadly Poison, Captain Greenskin and other weapon buffs. It was too powerful until Leeching Poison was nerfed to give temporary life steal instead of permanent. Kingsbane will be a big loss for Rogue.
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