I have over 5000 points of chaos marines, so I change my lists quite a bit between games. There are some units that see use more than others though, either because they are painted, they perform well, or I just like them.
I always bring a squad of daemons for shock assaults. The ability to charge after deep striking has always served me well. When a rhino carrying 10 marines is suddenly carrying 10 daemons as well, its pretty shocking. When I had more daemons I could drop up to 30 from one icon, and they always did well.
What is your 'core'? Whats units do you always bring? What performs well? What sucks?