Singers:
(or opera singers, if you insist.)
My favorite male voice is Placido Domingo.
I was blessed with three occasions to hear him live,
once in Tosca with Washington Opera after I stood in line for about
7 hours for a standing room ticket in November, 1988,
once in concert in Toronto's Maple Leaf Garden in 1992,
and once at LA Opera for a concert version of Wagner acts
with Maryinsky/Gergiev in 2000.
I enjoy the many Domingo CDs I have (almost) as much.
Most often,
if there is a recording of a particular opera with Domingo in it,
that's the version I get.
I also like Ben Hepner,
Bryn Terfel,
Richard Margison (I heard live, in Mefistofele with Royal Opera, 3/98),
and Lawrence Brownlee (I heard live, as a guest performer
in Met Audition Final Concert 2011).
After I heard Rene Pape overshadowing Hepner mainly for whom I bought
the Tristan und Isolde DVD,
I have seen him in Die Zauberflote (DVD) and Boris Godunov
(MET HD in local theater), I became a true fan.
Among the baritones I heard live,
Sherill Milnes (I heard at the University of Maryland vocal
festival in 1990?) and Paolo Gavanelli (I heard him performing
the title role of Rigoletto at Dallas Opera on April 2, 2011)
impressed me the most.
My favorite female voices are
Renee Fleming (absolutely beautiful in live performance in Rusalka with
Royal Opera in 7/03, far superior experience to her recording of the same!),
Frederica von Stade, Cecilia Bartoli,
Kathleen Battle, (all heard live)
Tatiana Troyanos, Barbara Bonney, and Judith Blegen
(unfortunately, never hear these last three live).
An unique case is Netania Davrath whose complete cycle of
Canteloube's "Songs of the Auvergne" on Vanguard is one of
my most treasured collection.