Control Priest with an infinite endgame combo. 

In nearly every matchup, you're looking to go T1 Test Subject, T2 Extra Arms.  Cleric + Pyro + cheap spell + Divine Hymn is your primary draw engine. 

At some point, your goal is to kill off at least one Test Subject and Convincing Infiltrator.  Then at 10 mana, play Undatakah + Seance to go infinite.  If you have two or more Test Subjects in your deathrattle pool, you'll get back 2 or more copies of Seance when Da Undatakah dies, and can then have access to infinite Undatakahs/Grave Horrors.

Swap out Convincing Infiltrator for whatever other deathrattle suits you.  The primary combo involves Test Subject.