I have browsed through this topic also (65 messages), and I was stunned.
Only a few people mentioned what I feel is very important.

A level of professional education is much higher now than 50 or even 20
years ago.  You cannot count how many orchestras are able to play "perfect"
i.e.  in tune and in time.  If they all get the same instruments - how
would you know the difference? Brass players want american instruments,
string players want european instruments, this makes a lot of difference.
The most important is (in the order) the person who listen's to the music,
and then the conductor.  I will give examples to explain what I think by
writing this.  I have bought recently four performances of Rossini's
"Barber of Seville", that what I am working on now.  If I would base on
orchestras names I would put the recordings in an order that is completely
different that the one based on my "listener" feelings.  And the winner
is ....  Orchestra di Rossini from Naples.  Second is Orchestra di Teatro
Comunale di Bologna.  Both are not in the top 10, not even top 100.  I
don't care.  I love the recording.  It is not perfect, and maybe that
is the point.  It is not perfectly in tune, and it is not perfectly in
time, but it lives.  The LSO and La Scala orchestra are not so good for
me.

the second "thing" is the conductor.  Do you know that very often the
conductor gets ONE rehearsal before the concert? THe orchestra knows the
whole piece, sometimes almost by heart.  The conductor is neede only to
tell them what are the tempi, dynamics of certain groups, some articulation
etc.  They do not have to practice.  Somebody was telling me that BSO
during some exam for conductors before Tanglewood were playing Brahms 1st
by heart, no stands, looking all the time at the conductor, and doing what
he wanted....  I could believe that.  This is what makes the performance
and thus the orchestra great.  A conductor that has a clear and consequent
vision of the piece.

This leads me to a conclusion that such a list is very difficult if
possible to make.  Perfection is impossible, and even if it was, nobody
knows what it is.

Another "anecdote".  Krystian Zimerman says he has no technical problems.
This is true.  Look what he has recorded.  He says that he plays only the
pieces he knows HOW to play.

Tomasz Biernacki
conductor http://www.dyrygent.prv.pl/
Poland