I've been a loyal wiki member for almost 3 months, and throughout my time here I have seen many guru's of the 40k universe. here's who in my eyes specialises in each field...
Vforvendetta - Lord of the Mechanicus (in the Counicl of Terra he is the Fabricator general of Mars)
Shas' O Kias - lord general of the Fire Cast (no part in Council of Terra, but leader of Tau Empire)
Algrim Whitefang - Lord Commander Solar (militant leader of Council of Terra)
Montonius - Inquisitor (inquisitorial member of Council of Terra, and thus most powerful man there)
Krannski - Fabricator General of Terra (the only other place I could think of, sorry mate) suspitiously the only techpriest that invents new technology
Registered Contributor - Scribe (hes got allot of work done on Horus Heresy)
Sereno Black Heart - Daemon Prince
Al'sorath - Representative of the Ordo Malleus (High Lord of Terra) never gives the full story, only snipets at a time
Please add more Users to the list...