Sondero
Don't you dare say you'd rather lose!
Which tier a Pokemon is put in is primarily based on their usage statistics. The more people use a Pokemon, the higher tier that Pokemon is put in. Ubers is the exception, because Pokemon in OU can only be banned to ubers if they're deemed too good by the OU council. There's also some other exceptions like this, like if a Pokemon is deemed too strong for a lower format, it'll be banned from a tier and only be usable in the tiers above, even if it has the usage for the lower tier. For example, if a Pokemon is deemed too strong for UU, it'll be put in UUBL, and thus only be usable in OU and above.How is it decided which tier a Pokémon is placed in?
You can read more about tiers and such here: https://www.smogon.com/bw/articles/bw_tiers, or here: https://www.smogon.com/ingame/battle/tiering-faq