  Weyl semimetals are extremely interesting. Although the first Weyl semimetal
was recently discovered in TaAs, research progress is still significantly
hindered due to the lack of robust and ideal materials candidates. In order to
observe the many predicted exotic phenomena that arise from Weyl fermions, it
is of critical importance to find robust and ideal Weyl semimetals, which have
fewer Weyl nodes and more importantly whose Weyl nodes are well separated in
momentum space and are located close to the chemical potential in energy. In
this paper, we propose by far the most robust and ideal Weyl semimetal
candidate in the inversion breaking, single crystalline compound tantalum
sulfide Ta$_3$S$_2$ with new and novel properties beyond TaAs. We find that
Ta$_3$S$_2$ has only 8 Weyl nodes, all of which have the same energy that is
merely 10 meV below the chemical potential. Crucially, our results show that
Ta$_3$S$_2$ has the largest $k$-space separation between Weyl nodes among known
Weyl semimetal candidates, which is about twice larger than TaAs and twenty
times larger than the predicted value in WTe$_2$. Moreover, we predict that
increasing the lattice by $<4\%$ can annihilate all Weyl nodes, driving a novel
topological metal-to-insulator transition from a Weyl semimetal state to a
topological insulator state. We further discover that changing the lattice
constant can move the Weyl nodes and the van Hove singularities with enhanced
density of states to the chemical potential. Our prediction provides a
critically needed robust candidate for this rapidly developing field. The well
separated Weyl nodes, the topological metal-to-insulator transition and the
remarkable tunabilities suggest Ta$_3$S$_2$'s potential as the ideal platform
in future device-applications based on Weyl semimetals.
