Cardassians are the only proven racists or biggest in ST as we seen how they treated the bajorans.They probably did same thing on numerous planets.They enslave or kill all prisoners and always acting so egoistic.
For example "Cardassians never make mistakes" Dukat said.
They always threat to anybody no matter even they are on much weaker side.
Cardassian empire is often called a Cardassian union but there is no evidence that they give other races a equal membership in their space.I mean just watch attitude of any cardassian and u will notice that they always look form above and they are acting so superior.
Also garak made a direct racial insult on two romulan soldiers when he is aboard on commanders Lovak warbird.