I recently purchased Djinn Oshannus and so far this card has been a game changer for me. I have been looking for a water tank and I didn't want to keep renting cards to level up.
Before Djinn Oshannus I was using the Spinkeback Turtle or Serpent of Eld as my tanks, now that I have Djinn Oshannus I only use the Spineback Turtle or Serpent of Eld if there is a issue with Mana cost.
If you choose Alric Stormbringer as your Summoner, you get +1 Magic Attack which brings the Magic Attack up to 3. Djinn Oshannus also has Void which is helpful if your opponent has monsters that have Magic Attack.
Overall, I believe this was my best purchase so far. If you have any other recommended cards, or comments let me know.