I would say trasero

In the Caribbean I always hear "culo" (which is the curse word version of butt) and nargas (which is basically butt). I can't speak for any other Spanish speaking country, but if you're in Puerto Rico or Cuba that's it

It depends on the register you want to use. See, as with English, Spanish has standard, nice and "bad" ways of calling parts of the human body. Also, you need to take into account the Spanish variant you want to use (we have many "valid" versions). Having said that, you may use these medium register (standard language) options for "butt": Culo (Spain) Nalgas (México, this is a legit term with medium register, but people in Mexico always seem to find it funny for some reason. 95% of the time I would not use this one.) Trasero (México, better term in Mexico. People of all walks of life will understand it and is 100% harmless. I will use this term 95% of the time.) In other countries... it will be better if people will comment on that. :)