Ummm...though question though, but i’m gonna answer it.
The tier which makes by smogon was mentioned very clearly and directly, like lapars as an example, it got good stat but outclassed by other water and ice type due to its movepool, typing, etc., etc..
Movepool was one of the most important thing, you need to have a better movepool for stalling, sweeping, tanking, etc., etc.. A lot of Pokemon has good stat but a lame movepool which makes them very frail.
The stat was another not that important as the movepool, but still important. Like pikachu as an example, it’s Atk, SpAtk and Speed stat was pretty nice with light ball, but it’s defensive stat sucks to lot that makes it almost ohko by everything :p It’s speed was also outclass by the others, which makes it more frail.
Another super important thing in either in game or compective battle, like Slaking and Rigigas, who has a super duper high stat of 670 was in a PU, why? Cuz’ it’s ability sucks. Tuant and Slow start makes them to the PU tier, tuant makes slaking could only move for 1 turn and do nothing in the another one turn, which sucks. And slow start halves it’s speed, atk and spatk stat in the5 turns which was still pretty long and most Pokemon couldnt takes so much hits from.
5.The Evs and Ivs
See Here too see how important it is.
Hope it helps!