I have not used any of the other resources that most people recommend so I cannot comment on those.
However, I am a big fan of the Navigate Test Prep app and recommend it to everybody preparing for the test.
https://itunes.apple.com/us/app/navi...606535337?mt=8
The app is free and you get 10 questions from each domain. You can buy all 600+ questions for $30 or buy each domain individually (Oddly enough, sometimes it is cheaper to buy them individually, the prices vary from $1.99 to $4.99 for some reason).
The reason this app is so helpful is that after you answer each question, it explains the thinking behind each answer; why the right answer was best and why the others weren't. It helps you to process the concepts behind the questions. There is an assumption that your knowledge is pretty solid, but it will help to highlight areas that you are weak. Once you figure those out, bust out your book and read that section over.
Also, it is not a monthly subscription, it is always with you and is SUPER user friendly. If you've got 15 mins of down time, whip out your phone and answer 25 questions. I am always using it as a refresher.
All levels of EMS are represented (EMR thru Medic) and each set of questions can be bought as an in-app purchase.