Here is the full list of all 20 Poker Night at the Inventory achievements. It takes around 20-25 hours to unlock all of the achievements on Windows.
Win a game on hard.
Win a hand with a Straight Flush.
Win a hand when you have three of a kind comprised of three kings (of any suit).
Win 12 or more hands.
Win a hand with a 6 card and a 9 card of any suit.
Win a hand with a Full House.
Knock out The Heavy after he's bought in with his special item.
Win consecutive all-ins.
Knock out Tycho after he's bought in with his special item.
Knock out Strong Bad after he's bought in with his special item.
Knock out Max after he's bought in with his special item.
Win three straight pots.
Win a full game.
Win a hand with Four of a Kind.
Check a pat hand.
Win a hand with a Flush.
Win a showdown with just a high card.
Win a pot with two or more opponents when you are all in.
Win a hand with a Straight.
Win a showdown where you and the opponent have same hand (pair or better) and you have a better kicker.