  Understanding the co-evolution of supermassive black holes (SMBHs) and their
host systems requires a comprehensive census of active galactic nuclei (AGN)
behavior across a wide range of redshift, luminosity, obscuration level and
galaxy properties. We report significant progress with JWST towards this goal
from the Systematic Mid-infrared Instrument Legacy Extragalactic Survey
(SMILES). Based on comprehensive SED analysis of 3273 MIRI-detected sources, we
identify 217 AGN candidates over a survey area of $\sim$34 arcmin$^2$,
including a primary sample of 111 AGNs in normal massive galaxies
($M_{*}>10^{9.5}~M_\odot$) at $z\sim$0--4, an extended sample of 86 AGN {\it
candidates} in low-mass galaxies ($M_{*}<10^{9.5}~M_\odot$) and a high-$z$
sample of 20 AGN {\it candidates} at $z\sim$4--8.4. Notably, about 80\% of our
MIRI-selected AGN candidates are new discoveries despite the extensive pre-JWST
AGN searches. Even among the massive galaxies where the previous AGN search is
believed to be thorough, 34\% of the MIRI AGN identifications are new,
highlighting the impact of obscuration on previous selections. By combining our
results with the efforts at other wavelengths, we build the most complete AGN
sample to date and examine the relative performance of different selection
techniques. We find the obscured AGN fraction increases from $L_{\rm AGN,
bol}\sim10^{10}~L_\odot$ to $10^{11}~L_\odot$ and then drops towards higher
luminosity. Additionally, the obscured AGN fraction gradually increases from
$z\sim0$ to $z\sim4$ with most high-$z$ AGNs obscured. We discuss how AGN
obscuration, intrinsic SED variations, galaxy contamination, survey depth and
selection techniques complicate the construction of a complete AGN sample.
