Unfortunately, there is no easy answer.  Gray gives a great definition of the ideal inbound link, but it doesn’t help to identify it.  The rub is that we do not know, with certainty, what the search engines consider ‘authoritative’.  For my money, the best way to determine this is to see what the actual linking patterns are between potential linking partners or sources.  Blogrolls are one wonderful source of info on this, since the blogs that are on a LOT of blogrolls in a space are probably the opinion leaders you are seeking.  Using a competitive analysis tool like Compete is another way to check out these nodes, by seeing for any given site what other sites are visited by the same people.  Finally, if you are a specialist in a vertical/business segment/locality, your instincts about who looks ‘authoritative’ is worth paying attention to–pretty often they will have the good inbound links you are seeking.  You just cannot ever pretend to KNOW what Mother Google is ‘thinking’.