So, this is my first actual rant since I joined devRant and I am not saying I am perfect either. Here goes nothing...
1. I honestly hate it when people use spacebar instead of tabs
2. People who have a bad indentation or no indentation at all (even though almost all IDEs have auto-indentation). The bad thing is when a person asks me to have a look at their code I always end up wasting time fixing the indentation rather the actual problem.
I love a properly indented code and that's one of the major reason I usually recommend Python to most people.
3. Lastly, people who leave lots of unnecessary empty lines. E.g.,

public class HelloWorld{

public static void main(String[]args){

System.out.println("Hello world!");

}

}