  We report a $5\sigma$ detection of the pairwise kinematic Sunyaev-Zel'dovich
(kSZ) effect, combining galaxy clusters in DESI imaging surveys and the Planck
temperature maps. The detection is facilitated by both improvements in the data
and in the analysis method. For the data, we adopt the recently released galaxy
group catalog (Y20: \cite{yang2020extended}) with $\sim 10^6$
robustly-identified groups, and construct various galaxy cluster samples for
the kSZ measurement. The Y20 catalogue also provides estimation of halo mass,
which further improves the kSZ measurement by $\sim 10\%$. For the analysis
method, we derive an optimal estimator of pairwise kSZ through the maximum
likelihood analysis. It also handles potential systematic errors
self-consistently. The baseline cluster sample, containing the $1.2\times 10^5$
richest galaxy clusters of typical mass ~$ 10^{14} M_{\odot}/h$ at typical
redshift $0.2$-$0.5$, rules out the null hypothesis at $5\sigma$. When fitting
with a pairwise kSZ template from simulations, the signal is detected at
$4.7\sigma$ and the average optical depth is constrained as
$\bar{\tau}_e=(1.66\pm 0.35)\times 10^{-4}$. We perform various internal
checks, with different cluster selection criteria, different sky coverage and
redshift range, different CMB maps, different filter sizes, different
treatments of potential systematics and the covariance matrix. The kSZ effect
is consistently detected with $2.5\leq $S/N$\leq 5.6$ and acceptable
$\chi^2_{\rm min}$, across a variety of cluster samples. The S/N is limited by
both the Planck resolution and the photo-z accuracy, and therefore can be
significant improved with DESI spectroscopic redshift information and with
other CMB experiments.
