What are the Current Must Have Cards?

I've been reading through some of the Deck Lists, and have noticed the same cards coming up time and time again. For example, Necro decks tend to field 4 Decay Spitters and Untamed Spirits, Haven decks tend to Field 1 Light of Tomorrow and so on.

So what is the list of cards to be on the lookout for? Cards you will generally want to play the maximum number of copies of in any deck you are allowed to play them in?