I tried PMing you, but it wouldn't go through.
I felt NBME 11 was the hardest for me. Here's how I did diagnostics:
1. Kaplan Diagnostic
4. UWorld SA1
5. NBME 13
Some say 13 is hard. Some say it is easy. I thought 13 was the easiest and 11 the hardest. It really depends on how you think and organize information in your brain. I also think 11 most represented the format of the NBME, so perhaps take 13 first, and save 11 for closer to the real thing?
Yeah, my bad, I meant USMLE, not NBME there. That'll teach me to post at 1:45 AM... I found 11's phrasing of questions and topics to be most representative of the USMLE I took. I thought 12 and 13 were very similar, so much so that I can't remember any significant differences really while I was taking them, but man I wanted to cheat while I was taking 11.
you could take an NBME, but i wouldn't start with 11.
you could also take one of the UWSAs, that will give you a good idea of where you stand.
or you could just jump into UW, since we have pretty robust data on what the % correct suggests about your preparedness (recommended)