From BMW, my nicest loaner was a 550i.
That said, when I was in high school, my mom got a 1993 Mercedes 500E as a loaner. If you aren't familiar with this - it was a limited edition beast that was hand built by Porsche. I spent the entire afternoon drifting around my neighborhood (like the idiot teenager I was).
I have no idea how or why she got that car as a loaner and I doubt I'll ever get anything much cooler than that one.