Hearthpwn

Avenge: This card is extremely good in both the early and late game. Playing it turn 1 is very powerful as it's synergy with very low cost minions in your opening hand can give you a huge early tempo push. It is also awesome coming out of Mysterious Challenger. Always keep this card.

Competitive Spirit: This is better coming out of Mysterious Challenger, but will also be awesome when you have a flooded board and total control. I usually don't keep this card.

Noble Sacrifice: Very situational, and more value from Mysterious Challenger. Never keep this card.

Redemption: Never keep this card. 

Repentance: Use this one against classes you know will drop a big minion. If you are playing against a paladin, and they have six mana next turn, use it to weaken Mysterious Challenger. If you are against Hunter, use it on Savanna Highmane, etc.

Abusive Sergeant: Standard in aggro decks. Drop this on turn 1 if you have no other options. Always keep this card. 

Leper Gnome: Also standard in aggro decks. Always keep this card.

Secretkeeper: The dream is to play two Secretkeepers and Avenge. This card excels in this type of deck. PLEASE NOTE: SECRETS GAINED FROM Mysterious Challenger DO NOT BUFF Secretkeeper. Blizzard isn't that stupid. 

Haunted Creeper: Sticky 2-drop that synergizes well with Knife Juggler and Defender of Argus. Always keep this card.

Knife Juggler: The combo with Muster for Battle is awesome, but don't hold it just for that on turn 2 ;). Always keep this card.

Shielded Minibot: My personal favorite 2-drop. Very hard for opponents to deal with. Always keep this card.

Divine Favor: Great card draw. Make sure to use any other cards that you plan to use that turn before using this for maximum value. Never keep this card.

Muster for Battle: Awesome Paladin card that has great synergy with Knife Juggler and Competitive Spirit. Always keep this card.

Consecration: Always seem to clutch a board clear with this card. Never keep it in your opening hand.

Defender of Argus: Awesome synergy with the whole of the deck. Avenge, Competitive Spirit, Haunted Creeper... you name it, it could use a Defender of Argus buff. You can keep this in your hand IF you have something playable on the turns before it.

Murloc Knight: Amazing card. With it you have the potential for a Murloc Warleader or an Old Murk-Eye. You can even get another Murloc Knight! Very popular card right now as it can flood the board fairly quickly. And god damn it he is riding a frog. I wouldn't keep him in your opening hand, however.

Piloted Shredder: Very sticky 4 drop. Keep him if you wish, however, in an aggro deck I like him later.

Mysterious Challenger: The reason that Secret Paladin is a thing. Very powerful to play on turn 6, and plus, Uther lights up like a Christmas Tree! Never keep this card.