  We present 16 new ultrabright $H_{AB}\lesssim25$ galaxy candidates at z~8
identified over the COSMOS/UltraVISTA field. The new search takes advantage of
the deepest-available ground-based optical and near-infrared observations,
including the DR3 release of UltraVISTA and full-depth Spitzer/IRAC
observations from the SMUVS and SPLASH programs. Candidates are selected using
Lyman-break criteria, combined with strict optical non-detection and
SED-fitting criteria, minimizing contamination by low-redshift galaxies and
low-mass stars. HST/WFC3 coverage from the DASH program reveals that one source
evident in our ground-based near-IR data has significant substructure and may
actually correspond to 3 separate z~8 objects, resulting in a sample of 18
galaxies, 10 of which seem to be fairly robust (with a >97% probability of
being at z>7). The UV-continuum slope $\beta$ for the bright z~8 sample is
$\beta=-2.2\pm0.6$, bluer but still consistent with that of similarly bright
galaxies at z~6 ($\beta=-1.55\pm0.17$) and z~7 ($\beta=-1.75\pm0.18$). Their
typical stellar masses are 10$^{9.1^{+0.5}_{-0.4}}M_{\odot}$, with the SFRs of
$32^{+44}_{-32}M_{\odot}$/year, specific SFR of $4^{+8}_{-4}$ Gyr$^{-1}$,
stellar ages of $\sim22^{+69}_{-22}$\,Myr, and low dust content
A$_V=0.15^{+0.30}_{-0.15}$ mag. Using this sample we constrain the bright end
of the z~8 UV luminosity function (LF). When combined with recent empty field
LF estimates at z~8-9, the resulting z~8 LF can be equally well represented by
either a Schechter or a double power-law (DPL) form. Assuming a Schechter
parameterization, the best-fit characteristic magnitude is $M^*=
-20.95^{+0.30}_{-0.35}$ mag with a very steep faint end slope
$\alpha=-2.15^{+0.20}_{-0.19}$. These new candidates include amongst the
brightest yet found at these redshifts, 0.5-1.0 mag brighter than found over
CANDELS, providing excellent targets for follow-up studies.
