I really like Wandering Monster in Arena, it often trades and gives you some sort of body on the board if you play it turn 2 or even 3. More than once I've managed to coin it out when my opponent has played a 1-drop which is a huge tempo boost.
Lost a game on the spot going second against a pally when I got Sewer Crawler as a random 3-drop. Don't know the actual average att/def for 3 mana drops though, off the top I'd say about 2.5/2.7. I think in most cases (except for preventing lethal dmg by a single minion) good old Bear Trap would have performed better.
Alexstrasza and Emperor Thaurissan are forever in my heart