Cheeseburger, but it often depends on the cheese (I hate those "we wanted to try something new and be different"-restaurants that put some disgusting cheese on it and don't even melt it).
There aren't many things I have such strict rules but if you mess with a perfectly good recipe just to offer something different (like those "European" burgers, yuck), you don't deserve to call yourself a chef. (the ingrediants may sound interessting and tasty on their own but together it's just awful)
Which brings me to...
"Never change a running system" -or- "A little experimenting doesn't hurt"?