I was thinking about this last night when I was studing for my last exam, B&R, and I'd have to agree with hnagia, Julian and co. when it comes to the most difficult. That being Architecture and Admin. Simple becaus it covers such a broad range of topics. It's a combination of all exams into one.

Ranking, for Oracle8i exams completed.

a. Architecture & Admin
b. SQL/PLSQL
c. Net Admin
d. Performance Tuning


Cheers