Almost always the front. I don't like them feeling like they're not even there, if that makes sense. It's kind of like when I worked in retail and people would not interact with me at all; it made me feel like I was just a machine and not valued as a real person.
I have sat in the back sometimes, like if I have luggage and they open the door for me, they usually open the back so I just sit in the back. Also when I catch taxi's with friends (which is rare) I would sit all in the back together.