Zubat (pokédex number 41).
The (national) Pokédex number of the Simple Pokémon™ are:
168, 257, 5, 160, 23, 83, 383, 97, 2, 593, 109, 131, 11, 29, 43, 60, 211, 19, 7, 73, 197, 3, 360, 193
Prime numbers except for 660 (Diggersby), 360 (wynaut) 168 (ariados), and 60 (poliwag);
Those four exceptions and the ones stated in the Hints are (tanks to @Will):
The orders of non-cyclic simple groups (without repetition).
So the Simple Pokémon™ associated to each letter is (thanks @Stiv) the Pokémon which name starts with that letter and
with lowest national Pokédex number equivalent to the order of a non-cyclic simple group. If no such Pokémon exists, the Simple Pokémon for a letter is the Pokémon with lowest prime national Pokédex number.
We fall back on prime numbers because they are the orders of cyclic simple groups; the listed Pokémons are "Simple" because the definition involves simple groups.
So the Simple Pokémon™ starting with Z is
Zubat (pokédex number 41)
- there is no Pokémon starting with Z which Pokédex number is the order of a non-cyclic simple group; and
- among all the Pokémon starting with Z 41 is the lowest prime Pokedex number.
A note about the Moderate Hint:
It could be the case that this list is not updated even when the Pokémon #1092 is revealed, for example if its name starts with P (for the same reason explained in the subtle hint). Just wait and we will see!