Along with the ones already recommended by other Listers, I'd like to
add some others(roughly in chronological order), as well as some IMO
interesting sidebars:

Monteverdi:  "Il ritorno d'Ulisse in patria".  Yeah, this is the least
popular of his three surviving operas, and arguably the weakest, but the
newish recording by Ensemble Elyma/Gabriel Garrido makes a very good case
for this work.

If you don't want to go for full-length Monteverdi operas, then I would
recommend as alternatives the longer narrative madrigals from the Eighth
Book.  These are "Il combattimento di Tancredi e Clorinda", and "Il ballo
delle ingrate".  The former is quite dramatic and has some pioneering
instrumental effects in the strings.

"La Calisto" by Cavalli.  The Jacobs/Concerto Vocale recording is my
favorite, with an instrumental realization that is even more colorful
than the Monteverdi operas.

"The Fairy Queen," by Purcell.  This isn't true opera, but rather
incidental music to a pretty mediocre adaptation of "A Midsummer Night's
Dream".  But the music (as much as a full opera in itself) is wonderful,
and recordings don't inflict the dialogue on you.

"La Purpura de la Rosa", by Torrejon y Velasco.  The first opera
written in the New World (1701), it's a different animal from most of the
continental European Baroque operas, in that it makes more use of strophic
writing and Spanish dramatic conventions (the libretto having been written
by Calderon de la Barca).  Recordings tend to emphasize the colorful aspect
of Spanish instrumental music of the time.  There's the version by the Harp
Consort/Andrew Lawrence- King, and a newer one that I haven't heard by
Garrido.

"Hippolyte et Aricie" by Rameau.  This was the first in a long line of
operas by him, written when he was about 50!  Another worth checking out
is "Les indes galantes".

BTW, some case can be made that Handel's dramatic oratorios are operas
in all but staging.  As long as you don't mind replacing your Classical
mythological characters with hoary Old Testament types....

Bill H.