  In this paper, we report a systematic study of the heaviest nuclei within the
relativistic mean field (RMF) model. By comparing our results with those of the
Hartree-Fock-Bogoliubov method (HFB) and the finite range droplet model (FRDM),
the stability and the shape of the heaviest nuclei are discussed. The
theoretical predictions as well as the existing experimental data indicate that
the experimentally synthesized superheavy nuclei are in between the fission
stability line, the line connecting the nucleus with maximum binding energy per
nucleon in each isotopic chain, and the $\beta$-stability line, the line
connecting the nucleus with maximum binding energy per nucleon in each isobaric
chain. It is shown that both the fission stability line and the
$\beta$-stability line tend to be more proton rich in the superheavy region.
Meanwhile, all the three theoretical models predict most synthesized superheavy
nuclei to be deformed.
