IMHO, asking a question like this and using the word "evil" is actually a very delicate thing- because of the very nature of what "evil" truly means. One could get very philosophical about that kind of thing.

People can wake up in the morning and decide to be "good", they can wake up in the morning and decide to be "bad"- but only a lunatic wakes up in the morning and decides to be "evil".

I say that in order to prepare you for my next statement- I don't believe that anyone in the Godfather films is genuinely "evil". Greedy, duplicitous, conniving, yes- but these are all elements of being "bad", which is to say, they did evil things for (at least in their minds) a valid reason, be it family, unquestioning loyalty, greed, the desire to amass power, etc..

A person who does evil things is not necessarily an evil person. It is the person who does evil for no reason other than to be evil, or "Evilness for evilness' sake", that is truly evil.

So there are no evil people in the Godfather films.

But if I had to choose, I'd say Luca Brasi, based on his backstory in the novel.

Sammy The Fish