The brightest quasars in the sky? How massive are their black holes?

The origin of black holes with several billion solar masses is an unsolved mystery. We aim to tackle it by discovering the most extreme cases, studying their masses and revisiting assumptions in their estimation. First, we present the All-sky BRIght Complete Quasar Survey (ALLBRICQS), which has completely mapped the bright end of the quasar population at all redshifts. Then we discuss our ongoing work to constrain their black hole masses, which includes (1) long-term reverberation mapping of some of the longest-lag quasars in the sky to calibrate the R-L relation at high luminosity; and (2) observations planned with the VLTI for BLR astrometry, in part for the same objects as in point 1. We will add interesting findings about the accretion states of outstanding objects.