It's hard to search through the entire (observable) universe, but the most luminous thing there is must on the other hand be easy to find. Can we now say that the most luminous quasar we know, is the (intrinsically) most luminous one that exists, or will we soon?
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As it stands I think the answer is no. The most luminous object in the (observable) universe is probably hidden behind many magnitudes of dust extinction. There are lots of faint, infrared-detected objects that may be extremely luminous galaxies in the early universe (and indeed some have turned out to be) but it is difficult to get their redshifts, so it is difficult to be sure we have a complete luminosity function. We now know that objects with central black holes $>10^{9} M_{\odot}$ already exist in the universe after a billion years or so. To get to that size there must have been an awful lot of accretion and an awful lot of accretion luminosity.
In addition, the most luminous galaxy/quasar is going to be a rare object and only ridiculously small areas of the sky are covered by things like the HST ultra-deep field. i.e. There is almost bound to be a more luminous one in some other patch of sky that has not (yet) been surveyed. The logistical problems of getting a UDF over the whole sky (or even a significant fraction of the sky) means that isn't going to happen any time soon.
Finally, the brightest quasar is certainly not the most luminous. 3C273 is quite a modest object, in terms of luminosity, but it is very close to us as quasars go. But perhaps you didn't mean bright in that sense? Brightness is normally reserved to refer to the apparent brightness. Luminosity is the extinction-free, distance-corrected power.
