NBME is the best one to do if you have to choose. very similar to real exam (I had some questions that vere exactly the same and some that were very similar). These tests also have a very good correlation to real board performance (see my thread on correlations for more info)
Kaplan CD is VERY hard. do not get scared by it and do not do it too close to your test date
Kaplan simulated is a bit easier but some of the questions are outdated and some answeres are no longer correct so watch out. This exam is old so its not too reflective of current USMLE trends. It is good practice for taking exams in a standardized setting though.