  We calculate the most massive object in the Universe, finding it to be a
cluster of galaxies with total mass M_200=3.8e15 Msun at z=0.22, with the 1
sigma marginalized regions being 3.3e15 Msun<M<4.4e15 Msun and 0.12<z<0.36. We
restrict ourselves to self-gravitating bound objects, and base our results on
halo mass functions derived from N-body simulations. Since we consider the very
highest mass objects, the number of candidates is expected to be small, and
therefore each candidate can be extensively observed and characterized. If
objects are found with excessively large masses, or insufficient objects are
found near the maximum expected mass, this would be a strong indication of the
failure of LambdaCDM. The expected range of the highest masses is very
sensitive to redshift, providing an additional evolutionary probe of LambdaCDM.
We find that the three most massive clusters in the recent SPT 178 deg^2
catalog match predictions, while XMMU J2235.3--2557 is roughly 3 sigma
inconsistent with LambdaCDM. We discuss Abell 2163 and Abell 370 as candidates
for the most massive cluster in the Universe, although uncertainties in their
masses preclude definitive comparisons with theory. Our findings motivate
further observations of the highest mass end of the mass function. Future
surveys will explore larger volumes, and the most massive object in the
Universe may be identified within the next decade. The mass distribution of the
largest objects in the Universe is a potentially powerful test of LambdaCDM,
probing non-Gaussianity and the behavior of gravity on large scales.
