APM 08279-5255 is definitely a brighter quasar in
absolute magnitude (at -32.2, a redshift of 3.87). It was
announced in August 1998. The Sloan Digital Sky Survey has
found other quasars at higher redshifts, though with a
sensitivity out to about 21.5 in apparent magnitude these
quasars may not be as bright in absolute magnitude. But
some of the recent discoveries with redshifts near 5.6
are almost certainly brighter.
HS
1946+7658 is at a redshift of 3.051, with apparent
magnitude 15.8 and absolute magnitude -30.3.
3C 273
(in Virgo and #273 in the third Cambridge Radio
Source Catalog) has an apparent magnitude of 12.8 and is
the brighest known quasar in apparent
magnitude.
However, APM 08279-5255