  We present a suite of high-resolution cosmological simulations, using the
FIRE-2 feedback physics together with explicit treatment of magnetic fields,
anisotropic conduction and viscosity, and cosmic rays (CRs) injected by
supernovae (including anisotropic diffusion, streaming, adiabatic, hadronic and
Coulomb losses). We survey systems from ultra-faint dwarf ($M_{\ast}\sim
10^{4}\,M_{\odot}$, $M_{\rm halo}\sim 10^{9}\,M_{\odot}$) through Milky Way
masses, systematically vary CR parameters (e.g. the diffusion coefficient
$\kappa$ and streaming velocity), and study an ensemble of galaxy properties
(masses, star formation histories, mass profiles, phase structure,
morphologies). We confirm previous conclusions that magnetic fields,
conduction, and viscosity on resolved ($\gtrsim 1\,$pc) scales have small
effects on bulk galaxy properties. CRs have relatively weak effects on all
galaxy properties studied in dwarfs ($M_{\ast} \ll 10^{10}\,M_{\odot}$, $M_{\rm
halo} \lesssim 10^{11}\,M_{\odot}$), or at high redshifts ($z\gtrsim 1-2$), for
any physically-reasonable parameters. However at higher masses ($M_{\rm halo}
\gtrsim 10^{11}\,M_{\odot}$) and $z\lesssim 1-2$, CRs can suppress star
formation by factors $\sim 2-4$, given relatively high effective diffusion
coefficients $\kappa \gtrsim 3\times10^{29}\,{\rm cm^{2}\,s^{-1}}$. At lower
$\kappa$, CRs take too long to escape dense star-forming gas and lose energy to
hadronic collisions, producing negligible effects on galaxies and violating
empirical constraints from $\gamma$-ray emission. But around $\kappa\sim
3\times10^{29}\,{\rm cm^{2}\,s^{-1}}$, CRs escape the galaxy and build up a
CR-pressure-dominated halo which supports dense, cool ($T\ll 10^{6}$ K) gas
that would otherwise rain onto the galaxy. CR heating (from collisional and
streaming losses) is never dominant.
