My test is in a few weeks, so I can't answer which ones are most predictive, but I can tell you what's out there. There are 7 NBME practice exams listed on their site - 3, 5, 6, 7, 11, 12, and 13. The higher the number the more recent. Those exams have former questions, I believe, so I'd say they have the most representative questions to the real exam. Unfortunately, you don't get solutions. You only get to see the questions you get wrong. Then I had to search the internet for answers.
Usmleworld is a question bank that is far and away the best resource (imo) in addition to first aid. It has 2100 questions along with 2 full practice tests. I recommend doing all of uworld and their practice tests. It has a ton of information, trust me. I was using usmlerx, but stopped a few weeks ago because uworld is simply better and I want to get through it. So far I've done NBME 7 and UWSA 1 (uworld self-assessment 1), and they both seemed pretty realistic, much better than Rx (though rx is a good study source).