Me cago en tus muertos = I shit on your ancestors / I shit on your dead relatives.
Me cago en todos tus muertos = I shit on all of your ancestors / I shit on all of your dead relatives.
And in the theme of insults from Spain, a loaded one is also:
Me cago en tu puta madre = I shit on your fucking mother / I shit on your whore mother
See, the thing with "puta/puto" is that it literally means "whore", but it's used to empathize cursings just like "fucking" is used in english. We're even misusing it by putting it before verbs, imitating it's use in english.
See, the thing with "puta/puto" is that it literally means "whore", but it's used to empathize cursings just like "fucking" is used in english. We're even misusing it by putting it before verbs, imitating it's use in english.
I think the Polish word 'kurwa' is coincidentally similar.
Me cago en tus muertos = I shit on your ancestors / I shit on your dead relatives.
Me cago en todos tus muertos = I shit on all of your ancestors / I shit on all of your dead relatives.
I've found this one even stronger and more insulting: Me cago en la sangre de tus muertos = I shit in the blood of your dead relatives
To add to the other comments- follar and joder are valid in Spain, however in South America "joder" means to joke around, whereas "coger" is used as a direct translation of fuck.
In Spain though, "coger" means to grab, and as you can imagine the different use for the same words makes people giggle in a variety of situations (such as when consuming foreign media )
That's not exact:
And in the theme of insults from Spain, a loaded one is also: Me cago en tu puta madre = I shit on your fucking mother / I shit on your whore mother
See, the thing with "puta/puto" is that it literally means "whore", but it's used to empathize cursings just like "fucking" is used in english. We're even misusing it by putting it before verbs, imitating it's use in english.
I think the Polish word 'kurwa' is coincidentally similar.
I've found this one even stronger and more insulting: Me cago en la sangre de tus muertos = I shit in the blood of your dead relatives
Is there a direct translation for the word “fuck” in Spanish? 
Adding to the other comment, "joder". Closer to fuck as a curse word.
What the two other guys said, but clarifications are needed.
However, there are execptions, as you can say both "que te jodan" or "que te follen" to say "fuck you" to someone in Spain.
Bonus way to say "fuck you": Que te den por culo = Get fucked in the ass
Why am I having so much fun explaning these?
To add to the other comments- follar and joder are valid in Spain, however in South America "joder" means to joke around, whereas "coger" is used as a direct translation of fuck.
In Spain though, "coger" means to grab, and as you can imagine the different use for the same words makes people giggle in a variety of situations (such as when consuming foreign media )