This is a close call. All three of these cards are good in their own way and could fit the current deck well. Considering there is a substantial amount of Paladin and Warrior players in Arena right now, I am leaning towards picking Acidic Swamp Ooze. You are unlikely to go wrong with the other two, though.
1. Acidic Swamp Ooze
2. Breath of Sindragosa, Big-Time Racketeer