That's a very interesting but tough question...

- Quenya certainly ranks highest on my list.  Tolkien tried hard to
make it as beautiful as possible, and apart from the inexplicable
omission of /Z/ he succeeded exceedingly well.

- Sindarin would also be quite high, though its mutation patterns freak
me out.  =P

- Klingon is dead ugly and uses an exceptionally stupid notation
(capital I to confound with l, but no lowercase i?!?), but its
complexity, internal consistency and originality are unsurpassed.
Great job, Mark.

- Esperanto should be commended for the noble idea and the general
uplift of conlanging into public attention, but the lang itself
doesn't do much for me.

As for conlangs from this list?  That's even more difficult, since I
know most of them only from a casual glance on the grammar page...  I
like many of them, but most don't strike me as specially noteworthy,
usually because of either plain-vanilla or unpronounceable phonologies
and either plain-vanilla agglutinating or drop-dead difficult gramamr.

- Tepa (by Dirk Elzinga) is one of the first conlangs I've seen on, and it thoroughly impressed me.  The phonemically
spartan but phonetically rich phonology is very cool, and the grammar
is so outlandish I never had the heart to take an in-depth look.

- KÚlen (by Sylvia Sotomayor) combines a very simple but original
phonology with one of the most daring experiments in grammar, the total
lack of verbs apart from four "relationals".  Great stuff. Congratulations.

- Nrit (by Shreyas Sampat) is one of the languages of Pii.  I've
watched the language grow on the message boards and in our IRC
channel.  I like its majesty and baroque complexity (though the
retroflex consonants spoil the phonology ;-).

I don't even quite know whether my own conlangs should belong on the list...

- Obrenje was supposed to be an a priori language which united
efficiency, fluency and aesthetics in one smooth shell.  I have made
many compromises since...  the aesthetics suffered for efficiency, and
the grammar is often reminiscent of IE langs.  Still, it's my most
complete lang, and I like it like an old friend.  Not brilliant, not
gorgeous, but familiar, trusty and proven.

A nuk tse blize jorue pri ruxal, cigo jocan ro sine sinar a
kikerdo tjes.
/a nMk tS# bli:Z jO"ru: pri rM"al "si:gO jO"han rO Si:n Si"nar a
ki"kErdO tjEs/
"And it happened when they were traveling from the east that they
found a plain in the land of Shin`ar, and they settled there."

- Jovian is only a sketch of a language so far, but evolving in
seven-mile bounds thanx to its Latin heritage.  I like its phonology
and orthography very much, with all the written diphtongs (ae, oe, ue,
au, ou, ei etc...) and the fluid mutation patterns.  The only thing
that bugs me about it is that it isn't a priori.  It feels a bit like
cheating.  =P

O doemu, dzidro bodilan augi mindraelis!
/A"DAjm "dzi:drA bA"di:l@"nawgivin"drajlis/
"O Master, I desire a bottle of mineral water!"

Izrimbaa~ /Mr`Mm"bA~_k/ was a very interesting experiment in phonology,
but I've never developed anything apart from the phonology.  Ah,
well... someday maybe...

Apologies to all whom I should have mentioned, but didn't.  =P

-- Christian Thalmann