  We present the results from an X-ray and optical study of a new sample of
eight extreme luminosity ultraluminous X-ray source (ULX) candidates, which
were selected as the brightest ULXs (with L_X > 5x10^40 erg/s) located within
100 Mpc identified in a cross correlation of the 2XMM-DR1 and RC3 catalogues.
These objects are so luminous that they are difficult to describe with current
models of super-Eddington accretion onto all but the most massive stellar
remnants; hence they are amongst the most plausible candidates to host larger,
intermediate-mass black holes (IMBHs). Two objects are luminous enough in at
least one observation to be classed as hyperluminous X-ray source (HLX)
candidates, including one persistent HLX in an S0 galaxy that (at 3x10^41
erg/s) is the second most luminous HLX yet detected. The remaining seven
sources are located in spiral galaxies, and several appear to be closely
associated with regions of star formation as is common for many less luminous
ULXs. However, the X-ray characteristics of these extreme ULXs appear to
diverge from the less luminous objects. They are typically harder, possessing
absorbed power-law continuum spectra with photon indexes ~ 1.7, and are
potentially more variable on short timescales, with data consistent with ~
10-20 per cent rms variability on timescales of 0.2-2 ks. These properties
appear consistent with the sub-Eddington hard state, which given the observed
luminosities of these objects suggests the presence of IMBHs with masses in the
range 10^3-10^4 M_Sun. As such, this strengthens the case for these brightest
ULXs as good candidates for the eventual conclusive detection of the highly
elusive IMBHs. However, we caution that a combination of the highest plausible
super-Eddington accretion rates and the largest permitted stellar black hole
remnants cannot be ruled out without future, improved observations.
