It would seem to me, if I may respectfully differ, that _what_ is the
best choice, as in, "President Clinton is to the US what Prime Minister
Blair is to Britain."  I admit that it is still awkward, but seems to me
to fit the sentence as given best of all the choices.  I agree, however,
that the analogy seems faulty.  My feeling is that _which_ would not
work unless preceded by _that_, as in, "President Clinton is to the US
that which Prime Minister Blair is to Britain."  In either case, it
changes the sentence from a simple analogy to a S-V-PN structure, where
the "what" clause is the predicate nominative of "President Clinton."
--
Aaron D. Profitt
The Gentle Misanthrope
"It is terrible to contemplate how few politicians
are hanged." - G.K. Chesterton
~veritas omnia vincit~
University of Kansas