I've been doing a few technical interviews in the UK of late, and anyone who mentions Active Directory on their CV usually gets this question - which is the most important FSMO role? In fact, there's a second part to the question - why? There wasn't any particular point to the question, however even if you don't specifically know the answer, it doesn't take long to work out which one it is and why through a process of elimination. Although the right answer is important, being able to deduce the answer clearly and logically demonstrates a good overall knowledge of what the roles are, why they exist, and gives a few talking points for a portion of the interview.
The answer is the PDC emulator. What struck me as co-incidence was that I was reading Brian Puhls blog yesterday and noticed he poses near enough the same question during his interviews. Brian is a domain administrator at Microsoft IT who certainly knows his stuff. So to find out why it's the PDC emulator, take a read.