First how nature and ivs matter for a pokemon.
IVs - IV range is 1-31, each iv point adds 1 stat point at lvl 100
Nature -Nature gives +10% boost to a stat to a stat and -10% to other(on pokemon description page the stat in green is + and stat in red is -), if a pokemon has 300 points in a stat, with 31 ivs and neutral nature, with +stat it will get 30 extra stat points, making it 330,pokemons with high base stat get even better boost in stat with benificial nature.
I will take example of a Staraptor for this one.
[highlight=yellow]Nature[/highlight] - Bad(bad nature for Staraptor are -att and -speed, including - timid and brave)
[highlight=red]IVs[/highlight]-
(only att and speed for Staraptor,other ivs dont matter, except hp, hp needs to be 20+ for all pokemons)
0-10 -fail
10-20 -very bad
20-29 -not good
30-31 -so-so
[highlight=yellow]Nature[/highlight] - Neutral(natures which are [highlight=orange]-sp att[/highlight] are also considered neutral for Staraptor, bcoz Staraptor is physical and dont need sp att)
[highlight=red]IVs[/highlight]-
0-10 -very bad
10-20 -not good
20-29 -so-so
30-31 -not bad
[highlight=yellow]Nature[/highlight] - Good(+att and +speed for Staraptor, except timid and brave)
[highlight=red]IVs[/highlight]-
0-10 -so-so
10-20 -not bad
20-25 -nice catch
25-30 -great
30-31 -epic
if 1 iv is epic and other is great with good nature-
if +nature -epic - pokemon is epic(if Staraptor +att nature and got 31 att ivs and 25+ speed ivs its epic att Staraptor , if Staraptor +speed nature and got 31 speed ivs and 25+ att ivs its epic speed Staraptor)
if +nature -great -pokemon is great(if Staraptor +speed nature and got 31 att ivs and 25+ speed ivs its great att Staraptor , if Staraptor +att nature and got 31 speed ivs and 25+ att ivs its epic att Staraptor)
if good nature - all ivs 20+(great/epic - speed and attack)-- very great
if good nature - all ivs 30+ - Best
I hope this helps in checking how good is a pokemon