  Jet substructure has provided new opportunities for searches and measurements
at the LHC, and has seen continuous development since the optimization of the
large-radius jet definition used by ATLAS was performed during Run 1. A range
of new inputs to jet reconstruction, pile-up mitigation techniques and jet
grooming algorithms motivate an optimisation of large-radius jet reconstruction
for ATLAS. In this paper, this optimisation procedure is presented, and the
performance of a wide range of large-radius jet definitions is compared. The
relative performance of these jet definitions is assessed using metrics such as
their pileup stability, ability to identify hadronically decaying $W$ bosons
and top quarks with large transverse momenta. A new type of jet input object,
called a 'unified flow object' is introduced which combines calorimeter- and
inner-detector-based signals in order to achieve optimal performance across a
wide kinematic range. Large-radius jet definitions are identified which
significantly improve on the current ATLAS baseline definition, and their
modelling is studied using $pp$ collisions recorded by the ATLAS detector at
$\sqrt{s}=13$ TeV during 2017.
