I'll just add my 2 cents

rule of thumb, most classes can go up 2 tiers (possibly reaching tier 0, see below for explanation on that) with the right optimization

tiers not covered by previous posts

tier 0 , technically possible but never meant to be played in a game (first that comes to mind is Pun-Pun). Trying to play such a character usually means a DM will just reboot the game and say ''no , you cannot do that'' or ''your character dies/goes insane/becomes unplayable for x reason''

One of the challenge of high level optimization is to reach tier 1, as high as possible, without ever entering tier 0 territory.